Raticate always could use Dark-type moves, but now they get STAB (Same Type Attack Bonus) whenever they use those attacks. While you'll hardly see these guys in the competitive arena, it does give them an added edge if you decide to train one for your adventuring team in Sun and Moon.
While today people aren't too thrilled with Rattata and Raticate, at one time they were very useful Pokemon because they can learn so many different moves through the use of TMs (Technical Machines) and HMs (Hidden Machines). Since this is still the case, it's nice to know that they can get an added Dark-type attack boost in case this is who you choose to round out your team with at any given point. Also, since today TMs are able to be used as many times as you like, you won't feel bad at all using them on an Alolan Raticate.