I remember reading a theory regarding Luke’s vague dialogue, saying that they did it that way in order to prevent people finding out about Luke’s involvement. If they just saw some random stunt actor, and his words pretty much just amounted to him being a Jedi who’s come to take Grogu away, then they wouldn’t think twice about it. But once they deepfake Luke’s face onto him and have him be voiced by Mark Hamill, suddenly that random Jedi becomes Luke.

I thought that theory made sense, but hearing that Mark was actually playing Luke on set, I’m not so sure anymore.