Imagine a dockside crane that moves about on a pair of rails parallel to a dock side (as here). The crane needs an electric power source, and therefore a building like this is put up on the dockside, to contain a big reel of power cable that unwinds, and rewinds as the crane moves up and down the dockside.
It therefore has an open doorway between the crane rails on the dockside (like this building - ie the lower door), and these buildings also have (as usual) an upper doorway, with a girder sticking out to enable them to winch plants, electric motors etc , in and out for maintenance.
These days, the crane power lead is often attched to the crane, rather than the dockside, so that teh cable does not scrape up and down the dockside.
Bullshi* - any takers ?