Well cowboy is an application library. So it is a bit in a different league. With cowboy you can start writing your business logic or application code directly and go to work. Nginx can do some of that with Lua integration but it is nice for serving static files and proxying to back-ends.

I could see for example nginx in front of cowboy. It would strip away ssl, serve static pages, maybe authorization/authentication and then proxy connections to cowboy servers in the backend for application logic.