Check out the tremendously helpful sticky by Bernie Brewer on washing yeast in the Fermentation and Yeast subforum.
The reasons I wash yeast is to lower my cost per batch, to always have the yeast on hand that I want, and because its cool - another interesting aspect to making handmade beer.
You may also want to check out 2 archived podcasts from the Brewing Network. Both were (I think, as its been a while) the "Brew Strong" show with John Palmer and Jamil Zainesheff. One episode on yeast washing, another on re-pitching yeast. Both extremely helpful on this topic.