The purpose of radiator valves is to control the heat output of radiators by altering the flow of hot water from the boiler to the radiator. The majority of modern radiators feature a thermostatic radiator valve, or TRV, on one side, and a lockshield valve on the opposite side.
Their use can be negated in rooms in which thermostats are present, as it could create a conflict in achieving the temperature you desire. Typically, TRVs will be more popular than manual valves, as their automatic settings can help with long-term energy savings.
For a deeper insight, explore our blog, Are radiator valves universal?