Probably one of two things.

If the battery bank that was originally installed in the boat was a dual bank, someone may have just left it the way it was as two banks of batteries.

I changed the wiring to one large bank of 8 6 volts. I had one of the batteries short out and had to remove two of them from the bank. If I had left the switch in I could have just switched from ALL to the good bank and took care of the problem at a later date. Instead, I was forced to take 2 batteries out of the bank manually ASAP. That's the only part of keeping the switch in that makes sense as far as I know.