Because it's an effective way to correct for some types of misalignment and has been proven over hundred+ years bikes have used dropouts.

I admit to being a bit careful to first suggest a method that is not easily reversable One could file the other dropout after finding that their assumption of needing to file the first one was wrong. This stuff sometimes reminds me of internet medical diagnosis. Without more data mistaken advice can cause more trouble then help. As the OP offered up more info the time for me to feel good about talking about a specific approach came to be.

I will give an example of why I can be hesitant to suggest filing drop outs. It was stated early on that for a 3mm rim shifting the dropout would be filed just under a .5mm. This is about half the amount that will do the job, thankfully the mistake was on the side of the "fence" that wasn't too far/much. But if the poster had said a different mistake and if the OP thought "hey the internet said to file off 1.X mm, I'll just go ahead" they wouldn't be happy to see the rim end up on the other side of the centerline.

But your post (#8) had what is now the likely technique of correction. Andy