Strain Relief Creamy white (internal thread) - M10x1
A strain relief ensures that the weight of the lamp is distributed safely and it reduces the pressure that the lamp or lampshade places on the lamp holder.
Without a strain relief, the weight of the lamp would only hang on the copper wires and you risk pulling the copper wires out of the connection points of a lamp holder / terminal block.
The strain relief clamps the cord, which means that the weight of the lamp hangs on the entire cord and not just on the copper wires in the lamp holder.