I’m sure he’s using it to match against his green slope book...which should be banned, so two birds!
With an assumption that the greens books used on tour ruffle a lot of feathers...
The detail in these books is amazing.
For years, hole locations given to tour players very clearly stated they were in steps or paces. No pretense to being exact.
At Erin Hills, the hole locations were very precise, and hole locations sheets very accurate. Which helped make the greens books even more helpful. I'm not certain in any way, but there haven't been too many (if any) tournaments where the locations were that accurate for the entire field. Augusta could be an exception, but there are no detailed greens books available like other events.
Getting the hole location as well as your own ball location as accurate as possible makes the greens books even more effective. Being off by even a little on some locations can change the read of a putt enough to even have a ball go opposite directions.
Greens books pro and con is not my point, but the better you can locate on them, the more precise your data. Fyi