The Steam platform is basically a store claiming to sell video games, but only offering them for rent. To detract from this flaw, it throws a few shiny things your way. If you don't mind paying full price for a lease or fancy shiny stuff, you'll probably like it.
While I wouldn't use it to 'buy' games, I deem it pretty convenient for sharing them. You only have to exchange account credentials instead of physical discs, which is quite handy when you have some far-off friends