The lower f stop lets you shoot in lower light conditions and you get shallower depth of field. The 50mm is f/1.8 and 18-55mm is f/3.5-4.5 (at least with Canon). Obviously you can't zoom with the 50mm prime but it's much faster and sharper. And of course it's dirt cheap.
Take a look at these threads:
Budget Lenses (under $200.00 US)
DSLR Lenses for Beginners