If you’re talking about pressure cooking pots then it’s a safety valve to prevent it from exploding. Pressure influences boiling temperatures, which in turn influence cooking results.
Otherwise, releasing the steam from the pot allows more steam to be emitted from the liquid into the air in the pot, which makes the dish dryer.
I usually find that it releases too little steam to prevent boiling over for something like noodles, but maybe I need to use bigger pots… Noodles you can leave the lid or “valve” open a little and watch the pot like a halk after the pasta is in but before it boils again (unless you added the pasta very slowly so the water doesn’t stop boiling, not for my patience). Afterwards, with a heavy pot you should leave the lid completely off and cook on a generous flame.
I would leave the vent open for boiling noodles, to avoid boilover. The lid you describe is a GREAT idea. I usually have to leave th cover slightly askew to achieve that same result. ALways use a BIG pot for pasta, ebcause you need to cook it in sufficient water and have enough room for the noodles to expand. Covering the pot keeps the water temp higher.