For one thing, image based distros are very convenient. If you tell someone “Just install Bazzite”, they will probably have a nice gaming experience without any tinkering, because everything is already set up for you ootb.
You have to understand the concept first. Fedora Atomic/ image based distros are built from top to bottom, not on the same level. If something changes from “above”, your install will change too, to an 1:1 copy basically.
Problem is, if stock Fedora isn’t allowed to ship/ doesn’t have some things pre-installed, it’s harder to iron out on the user level, e.g. by negatively affecting update times.
uBlue is basically a “build script”, that takes the upstream image, modifies it, and redistributes that with the changes included.
In that way, the image from other users is the same as yours, with the same bugs.
This makes it more efficient and user friendly.
It also allows devs to make their “own” distro with only their changes included, while offering a very solid base they don’t have to maintain themselves.