I can't answer why there are so many invitational, honorary, and side bodies in the York Rite. I can't answer for everyone, everywhere, but in my area, the bodies meet and have activities. The biggest thing (to me) is they all tell a different story. As previously noted, many of them are honors/awards bestowed on members who have presided over various bodies. For example, the Order of High Priesthood and the Silver Trowel don't meet regularly. It's a once a year thing during Grand Lodge week where those orders are conferred on Past High Priests and Past Illustrious Masters. KYCH is recognition of having presided over all four bodies.
One of the most active, and my favorite, in our area is our York Rite College.
If you add up all the various York Rite orders, bodies, etc., it probably comes out very close to the number of degrees and honors in the Scottish Rite, so we'll call it even.