The theory is when people "like" your page all of their friends will see their activity and possibly check out your page or even possibly purchase your products/services. Essentially it's suppose to be "word of mouth".
However if I'm being honest I rarely if ever click on a page simply because one of my "Facebook friends" (liked) it and I never accept invitations to play games either. My hunch is there are a lot of folks like me out there.
A lot of larger businesses offer "incentives" to get people to (like) their page. They may offer discount coupons or a chance to win some type of prize by a certain date. I'm still not sure how many "likes" convert to sales. As I said I've never become a customer due to someone liking a page on Facebook.