Hissing vent on a steam heating pipe
The vents on a steam system, by releasing the air, allow the steam to rise. Often, the risers far away from the boiler will have their own vent to get the heat there quicker. If you capped it it would probably take longer for that part of the house to get warm. From an efficiency point of view you want that air purged ASAP.

You can balance a system with adjustable vents.