It is a leftover from European vehicles, and perhaps US vehicles, where the vehicle could travel from the UK (miles) to mainland Europe (kph) or vice versa. Likewise, in the USA it's mph and in Canada it's kph. Hence it makes it easier to read the speed without having to decipher the smaller increments.
It also means there's no need to have both mph and kph on the speedo although they may still print some of them with this.