Just saw this. In a lot of systems (like the one I do) slaps are much preferred. There are several reasons. First off is that the mechanics of slapping are much like the mechanics of grabbing, so downward blows can and do have a lot of body mass behind them without much work. Also, striking hard things is apt to break bones. The litmus test is how hard you want to strike a brick wall. You can slap one much harder than you can punch or even elbow it.
Finally there is what blows do. I do a classical (Japanese) jujutsu system. in Ye Bad Olde Days everyone clanked around in armor, so punches were a terrible idea. Slap, however, could knock people off balance or derange their structure making them ineffective. These also work against much larger people (that 6’ 8" sumo wrestler with 200 lbs on you). Strikes in other words, set up other things, like takedowns, throws, locks, chokes etc. and the like rather than doing the damage themselves.
Oh and ever hear of the Ottoman Slap? That was the preferred hand to hand combat techniques of the Turkish army and was very much feared. Looks probably a lot like this:
Note that the guy who did it is a pro. He turned the receiver on his side to prevent choking in case he vomited while unconscious.