Why is that ring insert is preferable than valve seat reface where servicing valve seat of aluminum alloy?

I don't think it is preferable. Sometimes it is necessary if the insert is seriously pitted or is loose in the head. Sometimes they can fall out of the recess in the head, and hang a valve open. If there is no other adverse condition, they can be resurfaced just like the old iron headed ones, and they will work just fine.