1.5m vs 3m

Can someone here tell me why and when to switch from 1.5m to 3m?! That's one switch on my telephoto lens I never touch and always stays on 1.5m.
The lens takes super sharp close range images, but for object further away not so detailed. Could it be that minimum focusing switch? Just wondering

btw; I am talking about my new 300 f/4L iS

It limits the AF from going past 3 meters(about 10 ft), so it takes less time searching. I use it when there is something like a fence between me and the subject. 1.5m is full throw. You can go to the closer focus range by manually turning the focus ring while in AF.
Nick

Funny, I just put down the one I got from school for the weekend when you posted this.