Oh, I interpreted design documents as general things - documentation included.

Plenty of things at my current job have formal mathematical proofs backing it and it's helpful when justification is explained like that.

Invariants in networking architecture i need to carefully manage etc.

Depending on the culture some of this stuff is also needed once you get into politics land and need to present your ideas because you know it's better, no? Ig at a large company with too many business oriented minded people this line of work would fall flat