I think the goal of Swift on the server, or embedded Swift, is mainly to offer Mac and iOS developers a way to write everything in the same language.

Right now, my Mac app depends on a few simple web services that are written in a different language. It would be neat if those services could be written in Swift, so that I could just use the language I already know instead of relearning how to do decode JSON in Python/Ruby/PHP.

Swift on the server doesn't have to become widely used. As long as there is a simple way to write basic web services, it is useful.