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You are touching on something that harks back to about 80 years ago...
When steam condenses, that means that it has given up it's useful (latent) heat. If you do not let the air back in, you will create a vacuum in that radiator.
Because steam goes from high pressure to low pressure, always, and because a vacuum is the lowest of low pressures, it would otherwise fill with steam.
<i>("Nature abhors a vacuum", my dad always said. Tossing that back at him as a personal statement did not get me out of pushing the Hoover around the house mind you.)</i>
This vacuum and immediate refill with steam may cause the room to overheat; the steam "just got done", remember. That vacuum also can, to a lesser extent, hold back some of the returning condensate, the "finger on the straw" concept.
Now, back in the 1920's or 30's, there were in fact vacuum vent valves that purposely did hold a vacuum. For coal-fired boilers in particular, this was a great way to prolong steam production, more gently, when the coal fire was dying. (There are far better explanations, but this is the gist of it.) It was a perfect match.
With the advent of on-off control (oil then gas), vacuum would be created without regard for air removal elsewhere. Pandemonium in there, I tell you. :)