It’s more about accuracy. Most designers specify an explicit spacing between the edges of the characters and other elements on the screen. If the devs can’t access those edges, then they have to adjust in order to compensate. So having accurate dimensions would reduce time to delivery.

Makes sense. I've never seen mocks that relied on this particular box.

The border box is just a visual aid. The real issue is with placement, ex: centering vertically. If you don't know the exact text bounds you will have off center text.

Ah. That makes some sense. I'm curious how often this comes up.