It helps avoid hard to read contributions full of redundant entries. Instead of having to tell people to remove redundancy when they PR and wait for them to fix things, the game just tells them not to do it when they are testing own changes.
It isn’t enforced for mods. If you want to make some local changes, it’s better to turn them into a mod - that way you can copy it to new versions of the game without having to locate and change things every time.