The load terminal would typically be used to turn something on/off on a schedule. Typically that would be area lighting turned on at dusk, and off at dawn. The controller may be smart enough to know the sun position based on date, but they may also just use the power coming from the panels to know that it is either light out or dark. Which can be useful if you would like your area lighting to stay on when it is heavily cloudy.
All that is just 411. You could use those terminals to turn an RV porch light on/off, but I've never heard of anyone doing that. Would be a nice feature on an RV used as a hunting cabin, just one that comes to mind. 99.9 percent won't use those terminals on an RV.
My controller allows me to leave the DC load output turned off always.
