Also, as mentioned, if the bow is not intended to have circular tiller, remember you still have to know what to look for.
Use it to look for side to side wiggles or twist, as Pat said.
Use it to compare THIS section of the limb to the same section on the opposite limb.
Use it to show they the limb is bending consistently where it should, and to prove that X section is bending less than Y section, as it should. Like, no marks, no marks, no mark, then a mark, and now I have to stop or I'll break the lead.......
It also helps you find small bumps or high spots. If the pencil is dragging and suddenly lifts at one corner, check out why.
I carry a crayon these days and where the pencil made a mark, I color the whole area. Then I scrape until the crayon is gone.
If you make one, you might as well make one long one and one short one.