Well, there's nothing in the Bible that says "Thou shalt fast." However, in my (limited) experience with it, it has served four main purposes for me.
1. Teaching me how to limit myself.
2. Showing me what I find important - last year I gave up coffee for 40 days and I was surprised at how much I really didn't need it. I love
3. Showing me where my priorities lie - I remember hearing a story about a woman who lost a lot of weight because she realized that she had been loving food more than God. When she put her priorities in order, she was able to change her life.
4. Putting my focus back on God - The last time I fasted from food I found that while I spent a lot of time thinking about it, I was constantly being reminded that I was doing this to honor God.
The hardest thing for me, though, has been making sure that I'm doing it for the right reasons. I don't want it to be just about making myself a better person, but letting God work in and through me.
But as Chellebert said, if you feel that God is calling you to fast from something, then go for it! Just pray about it first.
-Amé