Automatically logs me out

What's the reason behind being automatically logged out every few days? Is it a security measure to help prevent other PC users from using your account or something? I was searching free agents and got a blank page because I had been logged out without knowing it.

I think the auto log out 'feature' is unnecessary nowadays unless there is a valid reason behind it. I can't think of any other sites I visit that kick me off like this other than my online email page.