I get what you’re saying, but let’s say I create a floating header, like on this website, and give it a height of 80px and put it on all thirty pages of my site. I then create a reusable group to hold all of the elements (menu, logo, etc.) and give it the same height of 80px and add it to each of the 30 pages inside of the floating group.
Now, what if there is a design decision to go from 80px to 67px? Now I have to go through 30 pages and update all 30 floating headers in addition to the group reusable element. It would have been much easier if I could have had a floating group reusable element as my floating header and only had to update once.