There is a long history of scientists developing theories for the quality and performance of optical systems. From George Airy in the 1830’s and his Airy Disc, the Dawes limit, The Rayleigh criterion, the Strehl ratio and on through to the intense calculation ability of modern day optical design software.
There are plenty of occasions though when something with a few microns flatness, and a consistent reflectivity is more than capable enough, and the cost of a laser grade quality component is wasted.
It might be a beam dump, a shutter mirror, a power meter reflector or a safety stop for example.
That’s why Laser Beam Products are happy to design a manufacturing standard below the extreme of laser grade that meets, both the performance target, and the budget.
Also I’d like to point out George Airy, William Dawes, Lord Rayleigh were all fellow Englishmen.