In addition to the above posts, that fasting is mainly a purification ritual, something also taught in religions which practice this discipline, is that by the art of giving up a physical or material necessity, it teaches the body, mind and soul to rely on something else for strength. For example, when fasting from food, not giving in to the physical feeling of hunger builds faith, strength (in a spiritual as well as physical sense), and perseverence.
Consider Ghandi's hunger strikes. Consider the many historical people who survived imprisonment and gave their God the glory.
The spirituality of eastern religions such as Buddhism emphasizes a connection between the mind, body and spirit. Fasting and avoidance of alcohol serves to purify the body, and allow for easier spiritual meditation and elevation. There is also the principle that the body is itself a temple, and therefore no drugs should be introduced into the temple that could alter the consciousness and make it more difficult to self purify.
Fasting is common in many religions, including Islam and Judaism, and is usually all to the same purpose, as a purification ritual that removes the sugar, fat and junk we put into our bodies just in the course of eating and drinking on a daily basis, again allowing people to, as they believe, more closely commune with God (or gods). In Islam, alcohol at all is strictly forbidden for just this reason.
Fasting, which involves not taking food for some specified period and avoiding food of certain type is a common practice among people in India, particularly those practicing Hindu or some closely related religion like Jainism and Sikhism. Fasting is practiced for several different purposes. It is done to generally cleanse the body and mind, as an austerity for spiritual development, or in observation of religious rituals, including way of observing some religious festivals.
I am not sure if their are any specific practices attached to avoidance of alcohol. In general consumption of Alcohol is not very common in among Hindus. Those, who do consume alcohol, are likely to avoid its consumption during fasting along with other items such as all non-vegetarian food.