There is a school of thought that having a smallish hole about 4 mm - allows water flow through the system so the thernmostat will open sooner as the heat from the hot water around the engine takes time reach the thermostat by convection. There hole is not large enough so as to bypass the thermostat.
The small hole also allows air to be bled thus reducing having an air lock, in fact the Rover V8 thermostat has a hole with a tiny float for this purpose.... but once the air goes, the float rises thus blocking the hole, hence contradicts what I've written above....