If your boat is an ex charter vessel, then it's quite possible that the switch you are referring to may be there to provide starting current from the house or domestic batteries via the engine start battery for use in event of the starter battery having lost its charge for whatever reason. I have such a switch on my boat which I installed myself for just such purpose, and where it was requested to be installed by the charter boat coding surveyor.
On my boat that switch remains normally in the off or disconnect position so that the domestic batteries are not accidentally discharged through a duff or flattened engine battery. I will look out the wiring diagram I made for it and post it here later on, or you can find it yourself in a posting elsewhere within the forum, though I can't remember offhand what the heading was for that thread.
PS. I've found the wiring diagram, sorry if it appears upside down, something to do with using an iPad!!
The switch I fitted is a battery isolater type and is located on the wood panel below the edge of the starboard settee. It requires a key to operate it, and that key, on my boat, was screwed to the woodwork under the seat of that settee in order that charterers did not turn the switch to the on position other than by a deliberate action to do so. It was also accompanied by a notice advising it was only to be used for emergency starting of the engine, and then to turned to the off position once the engine was running.
