They are reprints of the booklets you would find inside each Kinkeshi capsule. Each wave, or part, has a corresponding pamphlet that contains info about the figures in that set, checklists of all the figures released to that point, etc.
Basically, they are companion pieces to each part, as well as advertisements for what other figures you might get if you pumped more coins into the machine.
I bought a used Kinkeshi 418 re-issue set a few years back from a board member, and I can confirm that mine did come with the reprints:
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