What subjects benefit from textbook editions?
Take undergrad economics. While standard micro- and macroeconomic theory likely doesn't change quickly enough to warrant a new textbook edition every few years, students might be... irritated... if recent economic events (the US housing crisis, the Great Stagnation, right now the ruble meltdown) were not reflected and discussed. 

Suppose the last example of a major crisis in your econ textbook (printed in 2004) were the dotcom bubble bursting in 2001 - today's college students were barely walking back then. This would be ancient history for them.

Yes, of course a motivated instructor could work with an older textbook and provide the updates based on his own notes. This is a lot of effort, though, and apparently few instructors go to this trouble.