Beyond the obvious, that there's a main battery switch there, no. I know more about the inside of a U-boat with great thanks to your drawings than from any other source.
What I can say is that a modern surface boat usually has a battery switch that isolates certain batteries (or in the case of a U-boat, would isolate entire banks of batteries) so that you can draw power from one battery while the other(s) are either being serviced, charged or restricted to other purposes.
For instance, on my boat, I'll usually start/run the engine with one battery while I reserve the other battery for the electronics. The reason for that is that it ensures that if I run down a battery through electronics, I'll have a freshly charged battery available still for emergency starting/navigating.distress call. On alternate days, I switch which battery does which job so that one is always being charged.
The main switch can also join ALL batteries together at once for a heavy use (off, battery one, all, battery two) but you usually wouldn't run both together since the fresher battery will always hog the charge from the alternator (magneto on a sub?) and not distribute it evenly over both batteries.
The 'off' position is used when the boat is in port and not being used because there is a tendency for boats (and this would be worse for a steel-hulled boat) of power leakage and galvanic action with anything in the water, so you turn the batteries off when not needed to minimize this risk.
That's also the reason for the zincs on the aft dive planes, rudders and prop shaft supports that we talked about in a past exchange.
I'd guess that the main switch on a U-boat would function in a similar way but be more complex since there are many more batteries involved and the systems are more critical. Probably it would have more settings and more possible ways of hooking up the batteries together?
One more thing is that every battery switch I've ever seen on a boat has always been a fire-engine red. Don't know if that would necessarily be the case with WWII U-boats, but it seems likely.