Fury is a boxer who wants to be mentioned in years to come alongside true boxing heavyweight legends. For that he will have to fight the very best of his own era and generation one after another successfully defending his titles. If that does not happen Fury will not be held in high enough esteem to be mentioned alongside legends such as George Foreman, Lennox Lewis, Mike Tyson and the great Ali.
I think he does not need to fight Joshua to prove anything but after beating Wilder he really needs to fight Usyk in order to unify the belts otherwise he will not be able to cement the legacy he craves.
For Fury, this is a guaranteed win. He knows to himself this is going to be a lopsided bout which means he is surely to win against Ngannou. Not to mention the money he'd get from the fight. Joshua is out of the picture already. There are stories coming out that Fury is avoiding Usyk as well, not sure if it's true but he could just set that fight in the future for now.
Winning against Ngannou is close to an exhibition fight for him but it will be a validation for Tyson to be the baddest mf in the world. That's enough reason for his esteem.