Someone has to know how to cure titan aids. Isayama lacks the courage and (more importantly) the sway over his editors to actually kill off Eren and Armin from titan aids.
Zeke is the genius (11/10 wits) son of a doctor who possesses both royal blood and a has canon contact with the titan science research division in Marley. That's a whole lot of plot bullshit thrown in his favour that makes him useful.
Zeke promised Eren that he would come back and save him, mentioning that they were both victims of their father.
Zeke is also nearing the end of his life (<1 year left) and is showing no ill effects from titan aids like Uri and Kreuger did.
It isn't canon that Zeke can cure titan aids, but there is some stuff there in support of the theory.