There seems to be two categories of make-up wearing women out there... Women who use primer and women who don't. The women who don't, do it because they've never tried it... once they do, they are forever switched to the other category.
Foundation primer is the best thing ever. It makes it so you need less foundation, it smooths out your skin for a better foundation application, and it keeps your foundation from turning weird colors (are you one of those people who gets orange foundation when you start to sweat?). Not all primer is expensive either. Go out there and find a good primer for you!