Why would you need it? You don't! But if you buy larger Victron Energy SmartSolar or BlueSolar Charge Controller (check each model for applicability) and you don't want to use your smartphone to check the controller's status, production or make adjustments, then you'll need this convenient little pluggable LCD display.
How do you use it? Just remove the rubber seal/plug you see on the front of your controller and plug this Control Display in place of that plug. You can toggle the Control Display to use the features described below.
Article # SCC900650010