The "in need" role gets a bonus, which differs depending on the roulette and your level, upon completion of the dungeon. While leveling, you get bonus EXP, and at cap, you'll get bonus tomes, typically of Law. Guildhests also award bonus GC seals. You can wait for the corresponding bonus to pop, but I don't tend to fish for it long. If you have something to do in the meantime, great, but if you have the time to get in, do your dungeon and get out, there's not a whole lot of reason to wait.