The principle use of the rooftop is as a helipad, with a staircase linking to the back side of the building. A water tower can be used to put out fires in an emergency.
In Resident Evil 2, a helicopter has crashed into the side of the building, and the water tower must be opened with the use of a valve.
In Resident Evil Outbreak File #2, this area is initially under attack from crows, though a police officer AI will fight them off. After Rita escapes the building, however, you will return to find him dead.
In Resident Evil 2, shotgun shells are among the possible items to be recovered from the downed helicopter.
Resident Evil 2
Water pressure valve
A water pressure valve. It reads: "Excessive pressure may rupture the water tank."