A DOF is a redundant and pointless role. And people who have exclusive DOF roles have worse skillsets than a manager or a coach.
And DOFs don't decide tactics - that's the manager's job. If a DOF decides tactics then we're in trouble becuase DOF's are shit who couldn't make it themselves in a proper job.
We know the club (and perhaps most half decent clubs) buy players as a collective decision by the footbal men at the club. Of course the financial guys are also involved with the cost and ROI aspects but the football folks must sign off on a player.
A DOF is (at best) another football chap who can weigh in with his knowledge on players but how does that make him different from other qualified football folks at the club? If anything the DOF will wield his power which will make things worse if the other staff don't agree with his choices.