The adapter allows you to use the QH on implements that might not have the correct top link height.
As to bushings. A CAT I QH has CAT II sized bottom hooks. A CAT II QH has CAT III sized bottom hooks. Etc., etc. It's my belief that this is done to create a "wear" point. By putting a bushing on your implement CAT I pin and hitching to a CAT I QH you have created a wear point on the bushing rather than the hook of the QH. If you were commercially using a QH on a BB for example, the bushing would wear out rather than the implement pin and/or the QH hook.