It will be an eternal rule for you: In the seventh
month, on the tenth of the month, you must afflict
yourselves and not do any work, neither the native
nor the convert who lives among you.
Leviticus 16:29
The inner purpose of fasting on Yom Kippur is to allow us to derive our vitality, at least for one day, from a higher source than physical food. Freed from the pampering effect of eating, we can focus on our higher, spiritual selves. This, in turn, helps us realize more greatly the importance of repentance and renders us more apt to regret past wrongdoing and make firm resolutions for the future.
Even from the purely physical perspective, fasting forces the body to live off its own stored reservoirs, which it has created from the food that we ate previously. In other words, instead of metabolizing the three lower kingdoms – mineral, vegetable, and animal – we are metabolizing ourselves – the highest, human kingdom. We are thus living for the duration of the fast off a higher level of life.
Emerging from Yom Kippur, we are, with G-d’s help, able to relate to the material world from a higher, more spiritual perspective.
--Daily Wisdom Volume 3