The idea is that you want to clean out or wash the detritus - trub, hop debris, dead yeast, etc - out of the slurry for storage. If you do not then there is the chance that that debris will begin to autolycise and impart off flavors to subsequent brews.
This thread http://www.homebrewtalk.com/f13/yeast-washing-illustrated-41768/
explains the process of washing in painstaking detail.
On Tap: 1. Kelly R. IPA, 2. Roter Hund Hefeweizen, 3. Bud Killer Blonde, 4. Red Dog Pale, 5. Roter Hund Oktoberfest, 6. Pumpkin Ale, 7. McRed's Stout (with new nitro system and stout tap,) Cream Soda, 8. ESB # 3, & 9. Ordinary Bitter.