Bonsai; roots

As past videos seem to have failed to convey this point. You do not cut the roots of a bonsai tree to stunt growth. This is the complete opposite of what you are trying to get of out pruning the roots.

That is the simple answer as to why you prune roots from bonsai.

The longer, more detailed answer is that you are doing to bottom of the tree what is done to the top. Removing unwanted growth, re-balancing the flow of nutrients and giving the tree access to more nutrition and water.

This is done by :

Improving the nebari which is the connection between trunk and roots.

Removing the tap root of trees.

Removing most of the substantial roots that would normally fix a tree in place.

Increasing the fine feeder roots.

This makes the tree healthier, grow better and makes it possible to keep it in the pot for longer before needing to be re-potted. This means more growth not less. All the signs of a healthy tree.