Sometimes you get bored of shooting hookers on the street, in the trailer park or on the moon, fun though it may be. That's no problem though, you can download more maps!*
There has been much controversy surrounding the sale of map packs, and downloadable content in general. Microsoft, for example, sold Microsoft points in certain increments, designed to make left-over points essentially useless. But let's not get map packs in general confused with project ten dollar. As for the map packs themselves, they're a money-making tool, and they work.
Some content (a lot of content) is already on the disc when it goes gold (gets pressed/printed, whatever you want to call it) leading to some (rightfully) angry gamers when they discover their sweet new multiplayer maps were on the disc they paid cash money for in the first place.
Sometimes, it's completely legitimate, with developers just wanting to make more maps for the players to enjoy - and make money too. That's kind of important, and you can't really fault them for that. There's a fine line though between being totally ripped off, and supporting the developers. Learn to know which map packs fall into which category. Vote with your wallets, people!