> What is the purpose of the jack connector ?

Short answer
6 V AC or 7V DC power when the board is used in a standalone splitter

Long Answer
The splitter PCB is used in standalone splitters and inside the SyMBplexer as it was easier just to make one board.
In the SyMBplexer some components are not needed as it can get the 5V power it needs from the black cable coming from the Fast Noxy card.
If the splitter PCB is used in a splitter case it cannot get power from the network cables as most of them were fibre optic so it was powered from a small plug pack which connected into the jack connector.
So that it was easy for us to source the plug pack anywhere in the world we designed the board so it could take AC or DC plug packs and not care about which pin was which. 
The 5V regulator chip 7805 is missing from your board and there is a jumper taking the 5V from the black cable to where the 5V would normally come out of the regulator chip. 

David