There's a limit to how far you can push your total desynthesis. Unlike class levels, you can't take every desynthesis to 100.

And unfortunately, the systems for what happens once you push your total desynth over 300 are finicky - and losing even 0.5 of a desynth you've maxed can take a lot of time and effort to restore. So for players who decide to swap desynths or just screwed up and tried to level multiple before realising the limitations, these books are a godsend.