Actions that serve characterisation, not characters that exist only to serve action.
Luke appearing to train Grogu doesn’t make sense within the story of this show? The appearance of pre-existing characters will technically always be fan-service to some degrees, but so far they’ve all added something to Din and Grogu’s story. Bo-Katan and Boba Fett allowed Din to see that there’s more than one way to be Mandalorian, and Ahsoka and now Luke adds to Grogu’s Jedi training story, while also staying true to canon. Luke was the most logical choice for the Jedi that heard Grogu’s call considering this takes place 5 years post-Endor, and when someone as important and popular as Luke returns you can hardly blame the filmmakers for making it exiting for the fans.