Just to post the full picture:
These are just starting points of where you may want to add or remove material. In my experience, I have a really wide forefoot but quite a narrow heel. For my own personal last, I had a lot of material removed at FIT 8, which helped for the back of the heel but only that. I didn't take into account that the area around the sides of my heel and ankles were also quite narrow and needed to be tapered inward substantially.
I highly recommend ordering test shoes for your personal last, I'm regretting not getting a trial pair and then some. I personally think that the price of a personal last should be raised to include fitting shoes.
In my opinion, this depends on the amount and what kind of changes.
Some might just need a little extra space at the ball or little toe, and for them, as Patrick noted, it's just an economical solution.
With many changes, the need for trial shoes increase.