Combined with plugins, this behaviour can have advantages, for example with thomthom’s selection toys, you are able to convert identical groups to components, by right clicking on one of them > group copies > convert to components, after having selected all instances in the same sub-menu.
As Box points out, as soon as you edit one of them, it becomes uninstanced (which I always thought is a pitty, cause it does even if you don’t modify anything. They should get uninstanced if you modify something, not just double click and exit)
It’s like a bag of crisps. You have 10 of them in the store. The moment you open one you have 9 + 1 that is different, even if you close it back up right away.