Attic Ventilation Systems

What is the purpose of attic ventilation?

It seems like a simple question, easy enough to answer. Unfortunately, all too often, that's
not the case. Most homeowners, and even some experienced builders and contractors,
believe the only purpose of attic ventilation is to remove heat that builds up in the summer.
 That is accurate.  But what that answer leaves out is just as important as what it includes.