Ever since Society quests started to get neatly divided this way since... ShB, but honestly the division already existed in StB to a lesser extent, I felt they exist for people who don't want to play specific jobs to inject XP into those Jobs without having to play them, perhaps the devs felt that would be a necessity when there are achievements and mounts behind leveling jobs, and so they wanted to give those players an out, if a much slower out than just queing into a dungeon or roulette with the job and actually playing it.

I am fine with them, it's busywork either way whether they ask me to run errands, or run errands but I slap down 2-3 mobs along the way.