It's something I never really understood. And it seems to be something that is actually pretty much an US thing. I don't see the same clinging to dress codes over here in Europe.
How the heck can it be important how someone dresses who is in no contact with customers? I can see the necessity of "professional" dressing when one has to do with customers. That's a given. You need to follow the rites of the human tribe. Dressing up in a similar way as the one you get into contact with makes him identify you as "one of his kind" and causes him to like you. He looks like me, so he's one of my tribe. That's deep in our ancestor's brain. That's why three piece suits are pretty much a necessity in management meetings because managers look at you and identify you as one of them if you're in the same three piece junk.
It's also, btw, the reason why techs don't like managers and why any tech dressing up as a manager immediately loses support with his peers. He's no longer "one of us". He's "one of them" now.
And no, I don't digress, actually, that's exactly the problem these things create. Because "business dress code" identifies a tech as "not one of us anymore". We not only don't want to wear that junk, we also don't like people wearing it. If anything, it alienates people.