All of us have been yelled at for slouching - my generation in particular. (I suppose we've given up on the current generation of elementary-aged kids, but that's another matter.)
It's not, as one might expect, to save one's back. Nay, the purpose of this life lesson is that it's a hell of a lot easier to clear a belt holster when you are sitting up. Next time you're armed at the computer, get that butt against the back of the chair.