Stupid question on 70-200 L lens

Going to ask a really stupid question now, but no one knows me so here go's
New to digital photography, just had a 70-200 f2.8 L lens, on the lens is a switch for minimum distance 1.5 or 3 my question is what difference does it make, I know what it does but why not have it set on 1.5 all the time

The idea is to stop "Focus Hunting". So, for example, if you're shooting something 3 metres away, but there's a blade of grass or something 1.5 metres away and in the foreground, the lens would try and focus at the blade of grass in the foreground (with the lens set to 1.5). If the lens is set to 3 metres it won't do that!!

Tom

If you have it set to min focus distance all the time it takes longer to aquire focus on objects further away.

Thanks alot for the quick reply, that makes perfect sence, just never thought of it, thanks again.