They allow you to hook up multiple powered accessories such as heated grips, heated clothing, aux lights, chargers, Sat Navs etc to the battery but through a switched relay.
That way your accessories power up and down with the ignition key.
No possibility of leaving an accessory on and draining the battery.
They're worth every penny, especially if your second hand bike already has one on when you buy it 😁.
On site bargain potentially available if you want one:
https://www.v-strom.co.uk/smf/index.php?topic=39787.0