As per this question. What are these numbers visible from the runway at Narita airport? A UK document is linked in the accepted answer but it doesn't list big number marking on the outside of the terminal. Surely such numbering is of little value to pilots- too coarse of location information. So who use such a gate number and how?
The signs on top of the jet bridges are useful in identifying your assigned gate. Without these signs it would be easy to misidentify a neighboring gate if it also had a ground crew ready to accept an airplane. They are also useful for keeping an eye on an occupied gate from a distance while you wait for an aircraft to leave. They are also illuminated which helps locating the right gate at night or in low light.
The electronic signs high on the terminal building are helpful to aircrew and ground crews. They often list aircraft info, departure/arrival airports and time to scheduled pushback.
The on ground painted lines are useful for initially lining up for a gate and in making the elaborate maneuvers required getting into certain tightly spaced gates. Once you get this close you are being guided by ground crew and have wing walkers watching your clearances. The final painted ground positions for nosewheel position (by aircraft type) are only useful to the ground staff guy with the wands -- you can't see them yourself when needed.
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.
Gate designation that are visible to the outside are important for a number of reasons. Mainly for Ground Support teams that service aircraft, fuel the planes, mechanics, baggage, and yes even pilots to use to find the correct gate to park upon arrival.