Because, if you have an Android phone, then you will need to get an iPod Touch. You wouldn't pay for two phones, if you were sensible. Why woudln't you just get an iPhone, then? Well, maybe because of a contract. Maybe you need an Android phone for some other reason. Regardless, if you can't get an iPhone, for whatever reason, the iPod Touch is there for you to use.
I don't really see anyone watching movies or listening to music on Android phones. Forget about all of your iTunes content and all of the apps. That's why I go the combo route. I have a Galaxy S for my web browsing and phone needs and an iPod Touch for movies, music, apps and games.