JSF basically enables you to develop a web application with only model objects (JavaBeans) and views (JSP/XHTML pages). With “plain vanilla” JSP/Servlet you’ll have to bring in a lot of code to control, preprocess, postprocess, gather data, validate, convert, listen, etc the HTTP request and response. And then I’m not talking about refactoring it to a high (abstract) degree so that you can also end up the same way as JSF does (just a JavaBean class and a JSP/XHTML page per use case).
I’ve posted a more detailed answer on the subject before here: What is the difference between JSF, Servlet and JSP?