What's the intended purpose of the Book of Regression?
1. Self-imposed challenge.
Do you want to kill Uber-elder on lvl 1 character? With some Books of Regression you can try.
2. Farm in low-level locations.
Some divination cards can drop in low level locations, but Drop Rate penalty may prevent effective farm. Books of Regression allow to keep character on specific level to avoid this penalty.
3. Leaderboards
Some players want to compete for a place on Labyrinth Leaderboards, some of them have level limits - 40 for Normal and 60 for Cruel.
In addition to the excellent suggestions by @arghtype @VanBuzzKill (lab is the reason for the season on the book) there is a very similar potential XP penalty for a given area relative to your character level.
See Experience penalties in PoE wiki.
If your level is too low or high for an area you do not gain the same rate of XP. This changes a bit at level 95 where the grind gets grind-y-er.
who would be interested?
- Racers optimizing their progression to be very min-maxed (they try to barely skate through some of those Act1-4 zones where the leveling gear is throwaway SSF anyway)
- if you find yourself hitting a wall and progression is stale something needs a second look this could be a small part