It will be revealed when Story is close to Ending
Most Readers think it's about the Adventure or Navigation or whatever they are lacking
But Oda is using it as a Major Foreshadowing
Their Mistake imo is helping Koby enter Marines
And as you can see now, Koby was kidnapped by none other than Blackbeard
It's possible that Koby's Sub-Plot will have Major Impact on The World
(Im not saying Koby will catch Luffy/Zoro, i'm talking about a very different Scenario)