The final function of modern Solar Charger controllers is preventing reverse-current flow. At night, when solar panels aren't generating electricity, electricity can actually flow backwards from the batteries through the solar panels, draining the batteries. You've worked hard all day using solar power to charge the batteries; you don't want to waste all that power! The charge controller can detect when no energy is coming from the solar panels and open the circuit, disconnecting the solar panels from the batteries and stopping reverse current flow.