Why do designers/manufacturers...?

"audiophile grade"

Good is good. Audiophile grade" is slang for stylish and expensive to boot. Style is very subjective and no two people are likley to value exactly the same style.

A different way to look at it is what "tweaks" do designers do with their designs that are built in already, might not be readily apparent or even advertised, and that potentially could render certain additional external tweaks irrelevant or insignificant?

That's why YMMV, especially when it comes to things like tweaks.