In this case it's really about perturbation: what changes if Y is substituted for its close analogue X? Why is a 4-40 screw used here, rather than a 6-32? Or M3? What, of all possible arguments, made this one the author's choice? (Is it really the best, or does it hide something inconvenient for them?) Why is the city here and not there?

You start to see the flavor. This form of perturbative or differential analysis can be extremely powerful. It can also be extremely difficult and lead one down many side routes... such as, say, chasing down the diameters of all inch standard wood screws, not that I've done that or anything, just to determine what would happen if that wood screw were replaced by a self-tapping machine screw....