Originally, if memory serves, H to H was instituted to give the impression there were more Witlesses than in fact there actually were, by Rutherford, who never went D to D himself !
From then on it was actually a fairly good way of recruiting, until the last 4 or 5 decades, when they kept it going to keep J.W's busy, on the Mouse Wheel. It meant, coupled with Meetings and preparing for them, that most never had time to stop and think about their beliefs.
Even if it tails off in many ways, the G.B will want to keep J.W's very busy busy busy, doing pointless stuff, for the same reason, keep 'em busy ! Keep 'em away from The Truth about the Troof.