The doors exist to ensure proper security protocols are being followed when nearing the reactor. The pit appears (to me) to be a large geothermal cooling vent for the reactor which supplies power to the Naboo capital city.
As for why they were there? Only George can tell.
What the hell were those doors for anyway? And who has a giant open pit. Also why the hell were they in an industrial site?
Well, giant open pits with strange lightning running through them seems to be a standard feature of any descent Star Wars room.
If I had to guess:
The doors exist to ensure proper security protocols are being followed when nearing the reactor. The pit appears (to me) to be a large geothermal cooling vent for the reactor which supplies power to the Naboo capital city. As for why they were there? Only George can tell.