Oda is going to use the Saint Shanks vs Kid and Law vs Blackbeard to show that Kid and Law became strong enough to hold their own, but ultimately ending up hyping Blackbeard and Saint Shanks before their confrontation
Law has the Wano and potentially Zou poneglyph copy
Kid has the WCI and Wano poneglyph copy
So it'd be used by Oda for Blackbeard and Saint Shanks to collect those poneglyph copies before their confrontation, and I suspect that Saint Shanks has the mysterious one, all of that potentially leading to Blackbeard collecting all the poneglyph copies after beating Saint Shanks