Because, for some jobs, thief and ranger specifically but there are others, devs used iconic weapons or abilities belonging to iconic jobs, doing some kind of mixed classes. What if my favorite weapons are daggers but at the same time i dont like mudra system? same goes for bow, what if i like the bow but i dont like the bard-ish play style? Doing branching jobs they would give me the option to choose. This doesnt mean that extra jobs will not considered imo. Not all jobs can come from branching, this apply just to some of them