Well for one thing, they need a state of the art nursing home for not just the GB but all those old timers that are high up helpers and Corporate.
This would include living quarters, dental facilities, PT department (physical therapy) eye clinic and a small surgical suite for minor surgeries (skin cancer removals etc) and other more practical daily life services such as laundry, barber shop, cafeteria.
Then there is the bunker aspect. This is the underground ventilated state of the art war room where they will cower while they wait to be raptured.
In addition to all of the above (its real purpose) there is its stated purpose and they need the requisite housing and offices for the legitimization of those aspects also.