also if there is a fire on the boat
you want to be able to disconnect the battery power. this is why battery switches should not be in the engine room (where the flames and smoke will be) but somewhere accessible.
switches on all batteries is abyc code. as is the location.
it also helps prevent any unwanted drain on the engine batteries if left sitting long term or at anchor
also makes the boat safer to have less live wires on the boat at any time. no need to have the swtiches on unless you are using the engines. especially when those ones are normally not fused.