CI Sessions - time_to_update

[eluser]simonmaddox[/eluser]
I figure there must be a reason for this, so instead of setting it to a insanely high number (or 0, I imagine) I figured I'd find out...

What's the reasoning behind time_to_update? Every x seconds, it'll simply update the existing session with a new session_id.

The only reason I can think of is to do with session hijacking, but a constantly changing session_id makes using it in other tables (as a foreign key) a pain.