Why are gate numbers also marked on the outside of a terminal?
Of course it's helpful to the pilots, and the ground crew, too. Even if you know the airport layout in detail, it's easier to go to the gate signposted as C27, say, than to go to the seventh gate on the left. Now remember that everybody who has ever flown a plane into that airport has flown there for a first time, and everybody who's ever worked ground crew there had a first week.

Just imagine the corresponding situation of a car park with somewhere between a few tens and a couple of hundred bays, where it's very important that you park in your assigned bay. Would each bay's number be written on it? Of course it would.