[Dead Parrot]

Another reason for an isolator might be to power something only when the alternator is providing charge. Whatever that blue(gray?) wire is going to across the top of the relay/fuse combo might drain the battery if hooked direct to power.

Chasing mystery circuits installed by a PO can be so much fun!

Also possible the PO was having a stealth leaky circuit and used the isolator to prevent having a dead battery every morning.