Short version is you can use root or jailbreaking to modify stuff you normally wouldn't be able to on your phone.
I can only speak for Android phones but these days their bootloaders are locked by the manufacturer right from the start, which means they can't be tampered with unless you unlock em. There are sometimes alternatives but usually the only way you can unlock them is if the manufacturer allows it.
Once you get it unlocked, you can install a custom recovery to flash (install) a root platform (Magisk, SuperSU} and from there you can modify and fix stuff like customizing your status bar, battery charging routines, YouTube premium without paying for it, and more.