The horizontal buttonhole keeps the cloth from shifting vertically, albeit slightly, thus maintaining the alignment of shirt's pattern across sides.
Sorry, guys - that's a nice theory which does not work in practice. The comparative force exerted by your trouser waistband on the left, or outermost, fabric of the center of the shirt by faaarrrrr overwhelms the holding power in a flimsy shirt fabric of one horizontal buttonhole which is usually 8" to 12" below the waistband. Again as I intimated previously, if it were the last buttonhole in an interlined (read "strong") tuxedo bosom properly located right at the waistband, it would serve such a purpose. Mind you, there's nothing wrong with it. There's just nothing right with it either.