Why do buddists fast?

Fasting in the monastic community is considered an ascetic practice, a "dhutanga" practice. (Dhutanga means "to shake up" or "invigoration.") Dhutangas are a specific list of thirteen practices, four of which pertain to food: eating once a day, eating at one sitting, reducing the amount you eat, on alms-round, eating only the food that you receive at the first seven houses. These practices are adopted by individuals voluntarily, they are not required in the normal course of a Buddhist monastic's life of practice. The Buddha, as is well known, emphasized moderation, the Middle Way that avoids extremes, in all things. Fasting is an additional method that one can take up, with supervision, for a time.

Not all Buddhists fast!  Austerities are being practiced in the 'lesser vehicles', the provisional teachings.  Ultimately, there's only one Buddhist law,the law itself, NAM MYO HO RENGE KYO.  The Law is flawless. The one and only rule is to chant this phrase.  This leads to enlightenment & unconditional happiness.  There's no need to fast and subject yourself to other kinds of austerity.  Buddha is Life itself!  (Food is Life, too!) The law of NAM MYO HO RENGE KYO is inherent in all living beings, all phenomena. It is the ultimate, underlying reality of the Universe.  All life is eternal and interdependent.  Respecting all life is the Buddhist way.  The 'Middle Way' is the freedom to choose whether to respond either positively, or negatively at each life moment.  Based on the practice of chanting NAM MYO HO RENGE KYO, one's resultant high life state allows you to make positive choices consistently, thus changing the direction of one's life in a positive way, allowing one to change your 'karma'.  For more information, google for the "SGI-USA" website.