In theory it's for experienced players to pick up to signify they're more willing and adept to answer questions for newbies and guide them through content.
However, the nature of MMO players tends to be selfish - and so they sought to motivate players to choose this role by giving them tangible rewards for doing it. You get a special icon, a mount, and certain other things; this means now, rather than being filled with skilled players who choose to become mentors to help other people, it's filled with people who become mentors because they wanted a fancy crown to make themselves look cool and a unique mount not everyone else will be able to get.
So, basically, square shot themselves in the foot by doing that and doomed mentorship from ever actually being useful. IMO they should strip those rewards from the mentor program, change the crown to a more humble icon (the little watering can one people have suggested in the past is cute) and keep holding mentors to task and strip poor mentors of their title because boy howdy have I seen more mentors who rage quit and display really caustic attitudes and behaviors in-game than I've seen mentors willing to help do things like explain mechanics and help new players improve at the game.