In order to heat anything by means of steam, air has to be eliminated from steam vessels and pipes.
(Steam air mixtures are at a lower temperature than steam at the same pressure).
This is done as the steam is admitted/generated to/in the steam vessel by means of an air vent which might be manual or automatic.
Manual air vents are just a simple valve closed by hand once a fair amount of steam is passing from it. Found on steam boilers.
Automatic air vents close by means of temperature pressure changes.
Found on various steam heaters and at key points in steam pipework

Conversely as the vessel cools the vent has to open to prevent a vaccuum being generated which might damage some components/prevent their proper operation.

Pix of automatic air vents:-
https://www.google.co.uk/search?q=au...2 gC8sQsAQIHg

These are intended for industrial steam, pressure cookers have a more mickey mouse device.

And for the truly brain dead here (and we seem to have plenty), a drawing of a pressure cooker with the AAV clearly marked as such.
http://chefslastdiet.com/wp-content/...er-300x225.jpg

Air elimination is a very large part of steam system design.
And no, it's not as simple as some ****-fer-brains seem to think it is.