What do they even do in that building? It's a cable - does it need to be manned? guarded?

Routers, the actual optical terminals, high voltage power supplies for the inline optical amplifiers, (Undersea fiber cables need to repeat the signal every few hundred km, and those repeaters need power) a surprisingly large number of batteries needed for the gap between power loss and the backup generator getting up to speed, a fair amount of empty floor in case they want to add more equipment later, etc. That 2,700ft^2 runs out fast.