While sometimes moves are hard to figure out why certain stuff learns it and others don't, Wave Crash is shockingly transparent. It shows up on Basculin and Empoleon(the only mons to get it in LA), a couple mons that suck(Floatzel, Bruxish), and Paldean Tauros, Quaqueval, Palafin, and Dondozo. So it's basically purely a way to give new mons a powerful move so that they'll be relevant. There's no greater flavor reason to consider, and conversely it doesn't end up on every new Water, just physical attackers. The question is "were these mons introduced after the move was and do they want a powerful physical water STAB"? If so, they get Wave Crash. Otherwise, it's Surf/Waterfall.