The load output is generally of very limited use on a
boat, but it will still supply
power (from the
battery bank) when the
solar output is zero. It will therefore
work at night.
The load output can be configured to switch on an
anchor light at dusk, turn off a fridge if the
battery voltage drops too low etc.
Some controllers will even allow the load terminals to be programmed with a sophisticated set of parameters, which can be useful.
Without additional
electronics such as a relay, the output
current is limited and the complication of the above options is generally not justified for most
marine applications. KISS. However, if you have a suitable application there is nothing wrong with using the load output.