I've found that as long as the bootloader can be unlocked and I can install a custom ROM, there's virtually no reason to obtain root. GrapheneOS is good enough on its own that I've had no desire to use root.
Some things are difficult to do without root depending on the ROM. Accessing displays' highest brightness mode at will, using a file manager that can actually see and manipulate every file on the device, using custom gesture utilities, altering over-underscan settings, undervolting the SoC, etc.
When people say "root", they usually mean "unlock the bootloader". "A device I can root" fundamentally means "a device whose bootloader can be unlocked".
Though, Google banning unlocked devices from using Google Pay was a really user-hostile decision.