Block Ads without a constantly running process by modifying the HOSTS file, OS Tweaks (L Speed, HEBF Optimizer, ect.), Better Customization (Substratum), more features you can use, Hack games with Game Gaurdian (Useful in games like City Racing 3D where you can hack the IAPs to get them free, because $100 for one car is too expensive), Bypass regional or device restrictions on the Play Store by spoofing a different devices specs.... and for my favorite root ability, ViPER Audio (Nothing sounds as near as good, not even Dolby Atmos)!
The list goes on and on... I love root. Most devices I use I root. However rooting voids your warranty. Also some devices have better development and are more stable, the Razer Phone is still new so it may be a while longer before we have more ROM options with root.
There are two types of root. System and Systemless (Recommended, especially for new users since a factory reset will often undo changes you made that may of bricked your phone when something went wrong)