The BeOS was written from the ground up to handle digital media, in realtime, on consumer-class hardware. It is a specialized tool which is very well suited for particular purposes.
Microsoft's operating systems are more general purpose. We believe the BeOS compliments Windows (in any flavor) very nicely, providing people with the right tool for the job -- a sharp knife when they need that instead of a hammer.
When you need to run Office, by all means use Windows. But when you need to manipulate 16 tracks of CD quality digial audio, you may want to choose the tool better suited for the task: the BeOS.