When Steve Jobs spoke about iCloud Monday, he said that Apple was going to demote the computer to be "just another device." So, rather than your Mac being the digital hub for your media and personal information, that job would be taken over by online services--specifically, iCloud. Given that now that many of us have not only multiple computers but also one or more mobile computing devices such as the iPhone, iPod touch, and iPad, this makes a lot of sense. Coordinating all your information between these devices has become a chore--particularly when you attempt to do it all from a single computer. The promise of iCloud is that syncing media and data will "just work." Just enter your Apple ID on your various devices and iCloud will make sure that all those devices have the most up-to-date content on them.
Of course, money could also be an issue. Having millions of people sign on for iTunes Match at $25-per-year is an attractive notion for Apple (even if Apple ends up sharing some of that cash with the record labels). And there's always the possibility that Apple would add other pay-for services to iCloud--subscription music streaming, for example.
Of course, there's also the bigger picture: By creating a system where all your computing devices communicate seamlessly and let you access your media on demand, Apple is making it even more appealing to stay in the Apple ecosystem and to buy even more Apple devices.