Does that mean there is no difference is legroom?
Only up to a certain point. Usually the divider can only be placed in a certain range and all the seats in this range have premium class pitch. The trick is to be in the first rows of economy where you get the extra seat pitch at no extra cost. On some A/C types you can see the limits of the class divider because there are sockets they fit into.
Apart from seat pitch the main benefit of short haul premium class is the possibility of coming back the same day and changing bookings as you please. Access to premium class checkin and security is also a very useful benefit, but frequent flyers have access to this anyway with any ticket. Access to lounges is generally a disappointment.
Onboard usually all you get is some dubious food that you don't eat and a free newspaper you read umpteen hours before on your iPhone. You might also get a free drink that you really don't fancy at 6am.