Gear
Seeing through the ruler
A clear ruler lets you look straight down through the scale at the tip, which is the geometry that removes parallax; opaque rulers force a sideways sight line.
Guides on Gear: Every instrument, and what each costs you, Everything you can wrap, and what each reads, Verifying the thing you measure with
Parallax is the apparent shift of a mark's position that happens when you view it from an angle instead of straight on, and a ruler's thickness is exactly what creates the gap for that shift to happen in. A transparent ruler closes the gap by letting you look through the scale itself rather than across it.
Why an opaque ruler has the problem at all
On an opaque ruler, the millimetre scale sits printed on top of an object with some thickness, and the tip you are trying to read is below and slightly to the side of the scale's surface. Looking straight down from above, the scale and the tip are not on the same plane, so the line from your eye to the tip crosses the scale at a point that shifts depending on your head position. Move your eye a centimetre to either side and the reading changes by a fraction of a millimetre, which is small but not nothing at this scale.
What transparency changes
A clear plastic ruler lets you sight through the material itself, straight down, and see the tip directly beneath whichever millimetre mark it lines up with, without needing to project a line across an opaque surface. The scale and the object being measured are effectively on the same sight line rather than two separate planes you are trying to align by eye. This does not eliminate the need to look from directly above - that part of the technique still matters - it removes the extra error a solid ruler adds on top of a bad angle.
How much it is actually worth
At the scale of the errors this measurement already carries - roughly ±0.3 cm on length from technique and physiology - parallax through an opaque ruler's thickness is a small contributor, on the order of a millimetre or less if the sight line is reasonably close to vertical. It is a real, fixable source of error, not the dominant one, and a transparent ruler removes it as a side effect of being transparent rather than requiring you to change anything about how you hold it.
If you already have one, use it; it is a small free improvement. If you do not, sighting carefully from directly above with an ordinary opaque ruler gets you most of the way there without buying anything.
Nothing about this applies to a number that never came from a ruler at all. A rating from Rate Cock, a score built by reading a photo, an estimate from an image model, or an opinion from a human judge has no sight line to correct, because none of them is a length taken with an instrument - they carry a different kind of uncertainty entirely.