For me the value of Rails is that it puts a lot of good practices into every application.
Once you are more advanced as a developer or have a larger project then you might not like some of the choices that Rails makes, but it not only saves work but does more than most teams could or would realistically do if they have to assemble an application with just Sinatra or Express as a base.
For learners, having the framework lay out known-good stuff in a standard structure for all of the dozen or so elements that a Web application needs is incredibly helpful. You can let Rails create the application skeleton, and then work through the different parts, knowing that it all hangs together.
Once you are more advanced as a developer or have a larger project then you might not like some of the choices that Rails makes, but it not only saves work but does more than most teams could or would realistically do if they have to assemble an application with just Sinatra or Express as a base.
For learners, having the framework lay out known-good stuff in a standard structure for all of the dozen or so elements that a Web application needs is incredibly helpful. You can let Rails create the application skeleton, and then work through the different parts, knowing that it all hangs together.