Originally Posted by Atli

Yes, it is a security measure, limiting the window for a session hijacking.

There are ways for malicious persons to obtain your session cookie, which gives them the ability to hijack an open session. The less time the session is left idle, the less time the hijackers have to hijack it.
Not exactly bullet proof, but in cases like these our options are kind of limited.

Aside from that, this is also just a matter of clearing up the unused session data laying around on the server. Session are only meant to be temporary storage, lasting a single "session". If you leave for an extended period, once you return and resume what you were doing, that would be considered a new session. How long that period is, that is up to you.