Decorators are used either to define a function's properties or as boilerplate that alters it; it's possible but counter-intuitive for them to return completely different functions. Looking at the other responses here, it seems like one of the most common uses is to limit the scope of some other process - be it logging, profiling, security checks, etc.
CherryPy uses object-dispatching to match URLs to objects and, eventually, methods. Decorators on those methods signal whether or not CherryPy is even allowed to use those methods. For example, adapted from the tutorial:
class HelloWorld:
...
def secret(self):
return "You shouldn't be here."
@cherrypy.expose
def index(self):
return "Hello world!"
cherrypy.quickstart(HelloWorld())
