Going with what I would do, I wonder if the designers, at the time a

was on the drawing board, thought to themselves, "ok, we've come up with what we think should have electrical protection/activation (with the use of aircraft style switches/breakers), but maybe we should put in 1 additional for the owner in case he/she comes up with a need for an additional circuit?".
ie. - it's there for whatever use will fit the sizing of the breaker.
Just musin' my thots ...
Bill
Bill & Sharon Cassedy
" Sunset Seeker "
1988 32' Fly Bridge
Twin 318 Cu In / 240HP Chrysler power plants
Raw water cooling
Freshwater boat