1. Marketing Shanks for the new movie.
2. Giving Momo a chance to defend Wano on his own.
3. A cheap & easy way to make Wano prosper.
It's really, really stupid. If Oda wanted to have Momo mature mentally as well as physically, he should've just done the obvious and had Momo hold Onigashima up over the Flower Capital for an hour in homage to what Oden did.
These last few chapters feel like a waste.