In pretty much every brand identity I've seen, there are graphs, geometric shapes and lines and grids, measurements (with whatever arbitrary base units) and whatnot, drawn all over logomarks and the logotypes. The most overused standard for measurement is probably the mythical [sic] golden ratio.
The ways the measurements vary, typically involving an arbitrary unit (usually that unit is a the length of a letter in the logotype) to determine the distance between the mark and the type, or the clear space of the whole logo; using neat angles such as 90deg or 45deg to determine the slantedness of elements; using a uniform grid, etc. By "precise measurements" here I mean "measurements strictly conforming to any base units once those units have already been determined (e.g. the x-height of the logotype, the width or height of a grid cell, etc.)", not necessarily "measurements that are specific down to mm or inches".
It's probably for absolute consistency and to help people spot counterfeiters, but it's not that easy to discern the precise measurements to know if a logo is legit. Like, how many times do you check if the distance between the mark and the type is exactly the height of the letter z in the type, for example?
Not to mention precise measurements aren't as easy to check as say colors, layout/alignment, or forbidden distortions or overlaying of elements (e.g. skewing the logo, or putting it on a busy background).
So why do designers feel the need to use precise measurements (lengths, distances, angles, grids, etc.) in their brand identities? Only for hard-to-recognize-at-first-glance uniqueness? To make the identities harder to fake? To make their designs seem more sophisticated and filled with design intent? Do there even exist designers who just eyeball 'em all rather than bothering with precise measurements?