A 7 pin to 13 pin electrical adapter is mostly used if you have a towbar fitted with 7 pin electrics for a trailer, but you want to attach a cycle carrier which has a 13 pin plug.  It can also be used to plug in a caravan which has a 13 pin plug.

A 13 pin to 7 pin adapter is used when you have 13 pin caravan electrics on your car, but you want to tow a standard trailer, or use a cycle carrier which may have a 7 pin plug.

If you need to tow an older type caravan that has twin plugs A 13 pin to twin socket adapter is used if your car has a 13 pin socket

These adapters are also needed if you have a detachable towbar fitted and want to tow an older caravan. This is because the electric socket on most detachable towbars folds away when not in use, so there is no room to fit twin sockets, so you would need the 13 pin electrics and the adapter as well.