Years ago, performers found that by tilting their speakers, they got a better projection to their audience. You see units with legs on each side that let the amps be propped up at a tilt (Fender, etc.), you saw "trolleys" that allowed 4x12's to be tilted (Vox). I don't know when the 1st 4x12's came out with the top two slanted, but that has pretty much been the standard half-stack cab for a long time.
When you wanted that "wall of sound", you bought the straight cab and made a full stack. I think most performers today separate their cabs when they get to a venue that requires two 4x12's. (and they don't stand right in front of eight 12" speakers blasting out sonic destruction). 8)