I'll bet on some cold days, your thermostat never opens.... you're just trying to bleed a little air.. lol
When filling an empty cooling system, the bleed hole allows air to get past the thermostat, which usually will trap an air bubble until the thermostat opens, the trapped bubble will usually "surge" a load of coolant out the rad filler, wasting a ton of coolant, but also allows the engine to overheat to some degree, while its waiting for that opening...
is just way better to vent first,, can also take the top (intake manifold hose) heater hose off, and vent there while filling.