You get membership to a group of people and organisations dedicated to keeping this countries ROW open and free for all to use.
You do get access to a database of PROW. but its not a guaranteed source of information and should NOT be used as a guarantee that your driving legal byways.
It lists footpath and bridleways as well so you need to make sure you check that the "Lane" on their database is actually a legal byway. Some thing that catches quite a few peeps out..