As seen in the picture, this is how the pockets are designated in golf. In that game, you're always playing your opponent very defensively, so you must know which pocket they're shooting for. The score board shows which pocket they are on (with tallying marks or numbers - writing "upper left corner" or "right side" is too cumbersome), so the pocket numbering system is crucial in order to avoid mistakenly selling out a pocket.
I'm not saying you *have* to use this system when calling your shot in 8-ball (or other games), but if both players know this pocket numbering system, then there's absolutely no confusion, so it's not a horrible idea.
I use this pocket numbering system almost every time I'm describing to someone a shot I had during a match, or when quietly explaining to a friend how I'd shoot a shot in a game that we're watching on the sidelines.