We use the word "stoic" or "stoicism" for these practices, a name that traces back to Ancient Greece. The Stoics, Στωικοί, were a sect in Ancient Greece who followed the teachings of Zeno of Citium, who believed that it was our mistakes, our poor choices, that resulted in negative emotions, and that it was important to use logic, knowledge, and ethics to prevent this to a level that you never experiences these. In later times, the term took on a different meaning, to prevent happiness, pleasure, and enjoyment, rather than anger, fear, and hate. In effect, the emotions seen by the original Stoics as positive and allowed were grouped in with the negative emotions, in the understanding people had, and have, of the term. This is not to say all those who were considered Stoics in later time would have agreed and didn't experience these.
Stoicism, referring to post-Greek stoicism, and things that could be referred to by the term in older times, has taken many forms.
We see John the Baptist, dressed in camel hair, living on only locusts and honey in the wilderness. Camel hair is very course, much like wearing burlap. I've worn a burlap robe. John wearing camel hair constantly would mean rashes and calloused skin, for the movement would never allow your skin to get used to it. Locusts and honey is of course a very limited diet. He was likely very thin and somewhat malnourished, as it would take too many locusts and too much honey to not be so. He was in the wilderness near the River Jordan, and the wilderness of Judea, modern day Israel, is very unforgiving, with wild animals and physical dangers, and hot sun beating down. He would have had leather skin, tight, wiry muscles, be thin and gnarled, a wild man. But people came to him to hear his message because he spoke with the Divine, and connected with the spiritual in a way few others could.
We see similar from the Desert Mothers and Desert Fathers of the following few centuries, who gave up everything and went to live out in the desert. This is what the part in Life of Brian, where Brian falls in the hole where a hermit with a vow of silence is living and spoke for the first time because of it was making fun of. The hermits went out to live in the desert and devote themselves to God, putting various restrictions on themselves. But people began to flock out to find them, as they were thought to speak for God because of their stoic life.
One such example if Saint Benedict, who, finding he had a large group of followers after trying to be alone, realized there was no order and it would go bad quick without some, so he wrote what is now called the Rule of Saint Benedict (possibly orally and later wrote down or dictated and recorded by one of his followers), outlining how the people should live, This became the basis for the monastic movement that followed, where orders and monasteries were established for people to be alone with God (monastic from monasticus, ultimately from monos, alone). Each order had vows, which were of a stoic nature. They varied order by order, and sometimes monastery to monastery.
Vows of chastity, vows of poverty, vows of silence, vows of temperance, were all to be found, and others. Each of these is a denial, a taboo. Chastity is to abstain from sex. Poverty is to abstain from ownership, in some cases everything but your robe or smock, a staff, and sandles (following what was stated in Mark 6:8-9: "He charged them to take nothing for their journey except a staff; no bread, no bag, no money in their belts; but to wear sandals and not put on two tunics.") and in other cases, just a limiting of possessions. A vow of poverty also sometimes prescribed eating only what was begged for, to be like the poor that are poor without choice or vow. Silence is not speaking, sometime like what was shown in Life of Brian, never speaking, sometimes only speaking at prescribed times, like meals, sometimes just a limiting of speech or of what could be spoken of. Temperance is the abstaining from alcohol, one of the least common, as the Eucharist used wine for the Blood of Christ, though some monasteries had vows that only allowed this communion wine, no other alcohol.
Later, there was a movement called Anchorism, where the devote who had taken no vows and weren't part of an order wished to remain in the presence of the Lord, so were bricked in to a portion of the church. They did not leave, and remained there until death, their needs for food and water being brought to them. It is a very curious tradition, as most other examples of living on church grounds before and after were that of a priest or friar or monk who was the caretaker for the building or site. Anchorism was laity with no tasks to do, living out life in a bricked in cell in the church.
One of the most wide spread forms of stoicism, is fasting. Fasting, though sometimes applied to other things, is very specifically abstaining from food, just as chastity is from sex and temperance is from alcohol. It is a very specific act, it is a temporary giving up, for a specific reason, not a purging of things to be got rid of completely, which is either cleansing or sacrifice, to me, cleansing when it is getting rid of the bad, sacrifice when it is giving of something valued. Fasting can be a sacrifice, and can be used as a means to cleanse, but it isn't the same as either.
Fasting has been around since time began, to all indication. It is used in may cultures before various rites, and has a direct line of practice and theology and lore tied to it, from Ancient Judaism through Modern Judaism and Christianity. It is one of the few traditions with that type of longevity with little variance. This is important.
From a more Craft context, considering a historical viewpoint, both modern ceremonial magic (meaning Dr. Dee and forward) and the examples normally looked at as witchcraft before 1900 like cunningfolk and the like, grew up in Christian context, usually what we now call dual-faith if not fully seen as Christian. Fasting has been an important idea in Christianity since the beginning, though its importance has faded more recently. As it was most often seen as a means to get closer to God, or to show suffering or purity to get God to answer prayer, it likely held similar importance in various magical practices, be they ceremonial or folk practices. As such, they were very likely used in the context we're talking, and likely as a means to either get closer to the spirits, earn their trust, open to them, or get them to do what the practitioner wanted. As such, I suspect it was used both for what I use it for, some spirit contact, divination, and personal soul searching, and for what CMs use it for, preparation for ceremonies or workings, among cunningfolk and other folk practitioners. I can also see the use of it after a working, for a time specific working, fasting until the time it will manifest.
Ok, let's look on a more intellectual level, leading to present time application.
Fasting, like any form of stoicism, is emulation of the physical. My favourite example is anchorism, where the seeker was literally bricked in to a church so they lived in isolation but in the presence of God (not the greatest description the way I phrased it, but sufficient). It is the denial of the "flesh" to encourage the "spirit", the breaking down of the ego to get it to step aside. To an extreme, of course, it leads to a dualistic view, that the flesh is bad and should be punished or destroyed, or to health problems. We need our strength and full health for some things. To quote a common paraphrase of Aristotle, all things in moderation. Via media. Used correctly, safely, and with wisdom, it becomes a means to temporarily put the ego and general desires aside by replacing them with a baser and stronger desire based on need, in this case, food.
This can serve several purposes.
First, the distraction from every day concerns allows us to gain a new perspective, seeing things differently. This can aid in decision making, in contemplation, in self analysis, and in helping us walk the edges between worlds.
Second, the weakness created opens us up, both to receive wisdom and knowledge, and for possession and similar work, as our barriers and boundaries are weakened, allowing what couldn't pass normally to do so.
Third, the weakness can help with entering a trance state, with all the goals and uses that includes, being very useful in divination and in spirit contact.
Of course, these imply the danger as well. The distraction can cause us to miss physical issues that shouldn't be ignored. The weakness can open us up to things we don't want in us, whether ideas and opinions and suggestions, or spirits that are not of the type we want to open up to. And a trance state and weakness can both be bad in certain activities. You should avoid fasting and driving. All things with wisdom, all things in moderation.