The only thing I know is that after rooting you can install custom roms. But rooting your device immediately voids your warranty and you can breach your phone if you don't know what you're doing. Not to mention that you'll have to manually install updates because the normal method won't work.
There's mostly 2 main reasons. The main one is to have complete access to the system files (so you can uninstall apps that weren't uninstallable as an example), and the other one as mentioned, being able to install custom operating systems (usually when official support is done there are devs that make custom roms so that old phones can still have the latest android version. Or just being able to use your fav custom rom 🙃)
Your phone will not be the same as before rooting - Warranty will be void, Knox will trip, security features may not work properly, lose access to apps like Secure folder, Health, Pass and any other apps that are secured by knox. Banking apps may not work. Once Knox is tripped, you are done. It can't be 'untripped'. Even if you were able to restore the original software and locked the bootloader, knox can not be restored back to the original.
