Where would a mobile naval base be useful?

How useful would a converted mobile naval base have been on D-Day?

It was poorly matched to D-Day's port-building needs but could have been highly useful in Crete or the Far East.

The answer separates a fleet-support base from the port and cargo-handling needs of the Normandy invasion. D-Day required facilities for unloading an army, so bringing a valuable mobile naval base into that operation would solve the wrong logistical problem. Crete or the Far East would make better use of such a vessel because established repair and support infrastructure was scarce. In those theatres a large converted ship could move essential fleet services forward without waiting for a permanent base.