Credit checks are about granting you credit--loaning you money. As such, they lower your credit rating for a bit. (Note that this does *NOT* apply to checking your own file, that is harmless.)

In general it's not a big deal but when you're planning on taking out a mortgage it's wise to avoid any inquiries because they're going to go over your file like a hawk these days.

The last time I applied for a mortgage I got a call from the underwriters about a credit inquiry--huh, I certainly hadn't applied for any credit. I did figure it out, though--it was when my mother put my name on her checking account. Fortunately, they accepted that.