It looks a bit less cluttered, compare e.g. “40.0 Mm” “40.0 x 10⁶ m” or “4.00 x 10⁷ m”. Plus I think that he took into account that he wasn’t lecturing future physicists but future chemists - in Chemistry you rely on those prefixes all the time, and for most stuff you won’t be changing the order of magnitude too much. (Major exception, pK-whatever)
It looks a bit less cluttered, compare e.g. “40.0 Mm” “40.0 x 10⁶ m” or “4.00 x 10⁷ m”. Plus I think that he took into account that he wasn’t lecturing future physicists but future chemists - in Chemistry you rely on those prefixes all the time, and for most stuff you won’t be changing the order of magnitude too much. (Major exception, pK-whatever)