No different than rooting any other phone.
I root to do specific things - those are my benefits of rooting a phone: Real (local) backups using tools like NeoBackup, which can easily restore apps and data. Full file system access so tools like Syncthing can write to external storage (not a concern on Pixel since it doesn't have external storage). Ability to hibernate any app, including system apps that love to relaunch for no good reason. On that note, ability to fully manage broadcast receivers and prevent any app from launching other than when you want it to run.
If there's not something for which you specifically need root, then you really have no reason to root.
Also, no reason to reflash a factory rom, you can root Graphene just like any other.