This is an adaptor for a bathroom sink with a square hole where the taps are fitted. My Mum had a plumber try to fit modern taps on to her bathroom sink with these holes. He used silicone to bridge the gap which meant there wasn't any surface for the nuts to lock onto and stop the tap moving. As a result the taps were spinning and kept coming lose. These adaptors modified the hole so that it was the right size and shape for modern taps and allowed them to tighten up without moving.