Support is something with a lot of potential to benefit a party and people like the idea of helping the party do better and get satisfaction that way rather than doing the most damage.

Support is kinda weak in this game compared to others due to how it is designed, but in FFXI, people would invite a BRD, COR or GEO specifically because of the buffs they can bring. In my static when I used to play, my COR would be brought because 2 of its rolls could greatly improve the rate they could weapon skill and it meant zerg mechanics were quicker. In exp parties it could actually give an exp buff. It could also improve accuracy (which is not an issue here) and that could make exp parties go a lot smoother.

Some support roles are good for crowd control, this was great in things like SWTOR, the dirty fighting build made this good fun. CC exists in this game, but is rarely needed.