These small pieces have been cut and labeled from readily available sheets. Having had them for many years, they get lots of use. They are a quick way to assess small measurements without having to stare and dwell over minute ruler values, or evaluate confusing visual input.

The product is TTC PSS5A 14 Piece Plastic Shim Stock Assortment – 5” x 20” Color Coded Sheets.

(Note that I also put a few thicker wooden examples in the upper photo.)

Some practical examples:

You can slide a shim under a straight edge or square to check how much a piece is flat or out of square. Then, you quickly translate the .004” gap to a few fine strokes with the plane.

Similarly, you can assess the sole of a new plane, or a worn wooden plane sole. 

How much gap of the tenon walls with the sides of the mortise? How far off square is a machine blade?

How much edge of a door frame do I need to plane away? (Or, how much did I goof up?)

Assess the amount of a sharpened curve (or accuracy of straightness) in a plane blade. Get the amount of the appearance. You can remember the appearance visually but also keep in mind the numerical amount for future sharpening.

Evaluate the flatness, or an appropriate curve, of a hand plane. Below: checking the desired inward curve of the sole of a Yoshihiro Yamamoto plane.

There are many other uses! The basic idea is numerically evaluating with the shim stock and relating it to the direct visual assessment. You are adding to your perceptive ability in your work.

Added: 

The fine reader points out (see comments) the less expensive stainless metal gauge sets available. I have long had similar ones (see photo below). Yes, they too are effective and handy, though I do not use them nearly as much. Somehow, I like the separate, colored, multiple in every size, and multi-reproducibility in every size for the plastic ones.

Still, you might like the $7 metal ones instead of the $45 plastic set for your purposes. Amazon, of course.