Why would you need it? You don't! But if you'd rather not (i) use your Victron charge controller's built-in bluetooth connection (many current models have built-in bluetooth) to monitor and adjust via smart phone, or (ii) connect to your controller by wired connection to your laptop then you'll need this convenient little wired MPPT Control.
How do you use it? Just connect the MPPT Control to your Victron Charge Controller's VE.Direct communications port using a VE.Direct Wire (not included--see related parts below). You can then toggle the MPPT Control to use the features described below.