Branch Circuit (those circuits that connect off of the main batteries) protection devices (fuses, circuit breakers) are used to protect wires and connectors from being damaged by excessive current flow caused by either an over current condition or a short circuit. Especially susceptible are light branch circuits that when subject to  a short circuit situation can start a fire.

Any piece of electrical equipment on board a boat might fail for any given reason... for example the most common being water exposure.  If a catastrophic equipment failure were to occur, unless the equipment has protection like a internal fuse (which most do not), then that piece of equipment  could short out the incoming battery power lines  and your batteries themselves.  Batteries hold an enormous amount of electrical energy  and with a shorted device sitting across its terminals, the only thing left to limit that power is the wiring itself. 10 or 12 gauge wire can easily melt and start a fire in such a situation.

ALWAYS PROTECT  A BRANCH CIRCUIT OFF OF A BATTERY!

If installing solar charge controllers, battery chargers, inverters, lights or any other product to a battery make sure it is protected with a fuse or  circuit breaker.