Does anybody know why there are 2 +ve wires to the battetry terminal on this vehicle?

I have had quite a few problems with the electrics for a while which relate to the modules under the rear seat (water got into the vehicle which took ages to resolve and soaked the area under the rear seat). The car is now pretty dry, though still damp, so I think the problems likely still relate to the modules under the rear seat, or the wiring in that area. Or at least I did, but am now having second thoughts.

The symptoms are mainly that when you first go to the vehicle when it's been stationary for a while (i.e. overnight) then everything initially seems fine, but sometimes very quickly, sometimes after a while, the radio looses power, the wipers stop working, the windows stop working, but the car still runs. The problematic circuits work and then don't, then work again, etc.

Earlier today, trying to resolve, everything looked promising, except that the c/locking wasn't working properly and seemed to be laboured. As I use the car mainly for short distances, and as I have been having the fan on full to try and dry out the interior (and wipers, and radio, etc) I thought putting the battery on charge wouldn't do any harm and it was as I went to disconnect the battery that I started getting second thoughts. As I took hold of the +ve wire to keep it steady while I put a spanner to the connection ready to start loosening, the alarm started making an odd intermittent single beep so I'm now wondering whether it could be related to one of the +ve wires connected to the battery terminal. Does one of these provide power to the ignition system and the other to other circuits?

Thanks in advance for any suggestions.