Seems to me that its substantially aimed at people who are not particularly programmers - web development for SuperUser.com perhaps? (c.f. the link to Scott Hanselman's blog in my comment to the question) And who don't necessarily want to be "developers" but do want to have some capabilities to go beyond plain HTML.
Its also aimed at demonstrating that there is a complete stack and further that you can deploy applications using that stack to shared hosting without incurring additional costs (which is a new feature enabled by the new release of SQL Server Compact Edition)
Last its an entry point for those who might want to be developers - if you're talking about "developing" a website or a web application its not always the case that the programming involved is complicated
Without playing - I need to set up a VM so I can play in a sandbox - its hard to go further but WebMatrix is composed of a number of very capable tools and therefore offers considerable potential to develop apps beyond what is possible "out of the box".