This cools the radiator and regulates room temperature.
Those who have them struggle to understand their key function, therefore setting the heating control to maximum. People seem to like their radiators as hot as possible and TRVs don’t allow this.
A disadvantage with the TRVs is that the boiler needs to be constantly on, so they can draw heat as and when they need it. This defeats the claim of energy efficiency.
TRVs are however perfect in a room that suffers from extreme heat. It could be south facing, or the radiator may be too big.
The TRV will keep this room cooler, when the rest of the house is heated.