why does my radiator valve have a hole in the gate?

Doing a bathroom renovation, and have removed the cast iron rad and its fittings, etc. I was inspecting the angle valve to see if it was worth cleaning up and reusing. the rotating gate has a small hole in it that would keep it from entirely blocking flow. I suspect this is original valve, as it matches other radiators, so mid 1920s. System was originally gravity hot water.

Is this to prevent hammer when the valve is shut, or to keep the siphon working when closed? or something else? Just seems weird to have a valve that doesn't actually shut off flow. Doesn't matter to us, as the valve will never be operated.

picture shows the hole, a 5/32 drill passes through, an 11/64 does not.