Mentioning Servlets seems pretty out of place here. The Servlet spec isn't a web framework its an HTTP middleware. Its more similar to Rack than Rails. Both GWT and Jersey are commonly deployed on top of Servlets.
Because in the past you had to use servlets directly, or JSPs that you had to do a lot of work manually to separate business logic from the presentation layer.
Source: I worked at a Web startup circa 2000 where we did exactly that. I think people sometimes forget how far we've come.
