It is an important part of the equipment.

Used to complete the missing attributes, elemental resistances, flask duration, life, defenses, increase elemental damage, and more.

There is a poe belt that gives an abyss socket, Darkness Enthroned and Stygian Vise.

The most expensive Path of Exile unique belts Headhunter, Ryslatha’s Coil and some rare items with influence conquerors, shaper, elder.