Many modern vehicles require a bypass relay for the towbar electrics to work correctly.
If you vehicle requires this relay then damage can occur to the cars electrics if one isn't fitted.

The 7 way bypass relay may be used on nearly all (12 volt negative earth) vehicles. This includes those that require a simple bypass function for circuit protection or to avoid confusing the bulb failure systems right through to those with part or full multiplexed and CANbus systems .

Some vehicles will work without the relay but as soon as you put a lot of load on the vehicles lighting circuits
they will shut down.

The bypass is designed to switch power directly from the towing vehicle* to the trailer or caravan road lights using very small signals
from the towing vehicles road lights system, without the towing vehicles check control sensory systems being adversely affected.
The logicon includes a built in audible device which buzzes when the towed unit’s directional indicators operate.

Also by following the operational switching instructions, the fog light on the rear of the vehicle can be switched
off whilst the fog light on the rear of the towed unit is switched on.