For Sinatra, it’s really almost like a wrapper around Rack. So you first need to ask what the point of Rack is. Rack is basically a specification for how a framework should return a result, it can use what’s returned with any web server that Rack supports. So it’s really a compatibility layer that allows you to choose your framework/server combination at will, without worrying about whether they’ll work together. If your framework is Rack-compliant, you should be able to deploy on practically any server via Rack.
Now, the thing is Rack is very low level. Frameworks such as Sinatra give you things like nice routing, helpers, before/after filters, and a lot more. You just need to look to the docs to see what you can get. Rails is much more featureful, and in many ways “magical”. That is, you might write a single line in Rails that ends up doing quite a lot, which for some is a good thing, and for some too magical. I personally prefer Sinatra for this reason, at least before getting a decent understanding of Rails internals.