If the solar module is sized too large, where the power output of the solar module is greater than the power usage of the equipment, then overcharging can occur which can damage the battery. The charge controller allows the solar modules to fully charge the batteries, then redirects the solar module current so the batteries will not be overcharged. Inexpensive simple straight forward system. Charge controller prevents battery from discharging back through solar panels at night or during cloudy periods. It also prevents the battery from being overcharged.
Equipment can use power from the solar modules and batteries at the same time.
Can be expanded for AC appliances with an inverter. Limited to DC appliances. AC is what most people use in their homes for TV's, microwaves, ovens, washers, and other appliances that plug into the wall. The higher AC voltage allows you to run longer distances with smaller gauge wire. The inverter adds more expense to the system.