Short answer: It sure does, but not in the way you may think.
Steam traps trap steam but that’s not all they do. They also pretend to be air vents whenever the system starts. And I say “pretend” because the traps usually aren’t venting the air from the system. They’re just passing it downstream to a real air vent that will toss it out. This is an important job because every steam system ever built is open to the atmosphere and filled with air on start-up. Steam is lighter than air and won’t mix with air, so for the steam to move, the air has to get out of its way. If the air can’t get out, the steam can’t get in, and that means cold radiators and unhappy people.