I noticed this recently when I was using a wireless connection that I didn't trust completely. I had the browser set not to remember my username/pass, and almost every time I tried to post it would require that I logged in again.
On my regular PC I have it set to remember everything, and I have probably logged in twice there in a years time and I think both of those were after clearing my browser cache to test something in Flash.
YES- THIS IS ONLY IF YOU DONT WANT IT TO REMEMBER THAT YOU ARE LOGGED IN, im a contractor and therefore use "hotdesks" so I don't want my details remembering, this is a setting the admins can change, it does release server memory (not much) I guess.
BUT I still would like it increased past 20 mins or so which appears to be the idle time..