Pokemon Sun and Moon: Because Nintendo thought Solrock and Lunatone needed an actual purpose in the game.

I literally cannot think of more arbitrary Pokemon. Magikarp is useless but its point is that you deal with it until it evolves into amazing Gyrados. I seriously. SERIOUSLY. Cannot think of more arbitrary Pokemon than Solrock and Lunatone. Sun/Moon is more Gen 6.5 than Gen 7. Maybe they'll introduce more relevant pokemon to be the legendaries, but I guarantee you Solrock and Lunatone are going to have some relation to the plot. Ancient power and whatnot.