In a nutshell, and somewhat simplified: it cleans up the /dev/ interface and turns everything into streams of characters and mounts. So rather than a bunch of messy ioctls and other out-of-band mechanisms it just works with streams. This is nice because it allows you to use remote resources as if they're local.

Distributed name spaces, distributed computing, compute servers, distributed file servers and so on.

Remember the SUN slogan 'the network is the computer'? Plan 9 makes that a reality.