Doesn't even have to be balance, though. There's a lot of small things that are just good ideas. Like, in Stellaris, there are various status icons on pops. In the base game, they all have the same colour. There's a mod that gives them different colours, so you can tell at a glance how bad they are. It's a good idea, so I install it. What's the problem with that?
Sometimes, it's also just things where it shows the game designer had different priorities. Like, Skyrim is a good game. But it shows that the menus were done for consoles more than PC. So I install a mod that changes font size to about five times smaller, puts more on each screen and gives more inventory sorting options. That's just a good thing for me. What's the problem with that?