Growing trees in the ground
 

I was wondering. I know that trees when grown in pots must have there roots
trimmed occasionally. I was wondering though why when trees are grown in the
ground people dig them up every few years to trim the roots. I would have
thought that letting the roots grow would mean that the trees trunk
thickened up alot quicker meaning it would have to spend less time in the
ground. I must be missing something obviously so i would appreiciate it if
someone could explain this to me. Thanks.