The load output is for exactly your example of a string of little dc lights. The controller has a programmable relay inside so that you can turn them on and off at specfic times, or at the very least a simple dusk-to-dawn operation so the small dc load isn't energized during the day when things are charging. But you aren't forced to use the dc-load on the controller.
USB on controllers. Ignore them. Just marketing. The danger is that if the usb outlet on the controller is overloaded, and is part of the main circuitboard, it can take down the whole solar controller. I always recommend simply ignoring the usb outlets on controllers.
So.. battery goes to battery connection on the controller - and you hang everything you want to use downstream off that. Except for if you want to power up a dinky little load with programmable on/off times.