User blog comment:Kaido King of the Beasts/Chapter 906: What is This, a Crossover Episode?/@comment-24850639-20180601152149/@comment-53539-20180601201613

I think it has more to do with JoyBoy and the Mermadi princess of old, due to this one seemingly bigger then Luffy's one.

Edit:

Basically my theory is the 2 hats was part of something the two had and when whatever plan they were trying to pull off occurred. The WG ended up with one of the hats secured away and the other was either lost or taken away and hidden.

The WG has one hat basically because of its meaning, destroying it is not an option for some reason so they keep it locked away instead. They don't want anyone to know about it, because if someone knows the reasons behind their existence, there will issues, especially as its a copy of the one Luffy is wearing.

Meanwhile the other Straw Hat Luffy has is being held by someone to protect it. As one wearer's time passes they pass it onto the other and ask them to protect it as its special, but they may not ness. know why. So the hat has been passed from person to person, each keeping the hat safe until its purpose will be served.

We don't have enough details yet so this is just a theory and a lot of missing information is involved so I wouldn't like to guess why their significance is.

At this point I wouldn't be surprised if it didn't turn out the original rulers of the One Piece world were the Fishmen/Merman races and Humans stole everything from them... Unlikely but I can believe anything.