When the God Valley Incident was revealed, a lot of people wondered why Roger chose to go there and fight against Rocks with Garp. All the players in that event seemed to have a clear reason. Garp had his duty as a marine (whether he liked it or not) and Xebec wanted to rule the world. However, Roger’s reason is still a mystery.
I believe the reason Roger went to God Valley (and perhaps part of Xebec’s reason) was for the Road Poneglyph located there. Now you might be thinking, “wait we already know where Roger got all 4 Poneglyphs: Zou, Wano, Fishman Island, and Whole Cake Island. How could there be one in God Valley?”.
Well, one of these locations is not like the other. Of the four Road Poneglyph locations, 3/4 are places that have some significance or relation to the Void Century or Joy Boy. The one place that doesn’t seem to have much significance beyond being Big Mom’s home base is Whole Cake. This leads me to believe that it is likely not the original location of Big Mom’s Poneglyph, unlike the other 3 islands, which seems to be where they were first left behind.
Now, I can only think of two islands in the New World that seem to hold a lot of significance in history: Elbaf, the island of giant warriors independent from the Emperors and World Government, and God Valley, a mysterious settlement of World Nobles. We can strike Elbaf off since Big Mom is unwelcome there, and would have likely kidnapped some giants for her country along with the Poneglyph if she invaded.
This leaves God Valley, a location where Big Mom was present 38 years ago and a place that likely is significant to the history of the World Government and therefore the void century. It also doesn’t seem to be a coincidence that Roger found the last island that can be located via logpose, Lodestar, exactly 1 year before this event and 39 years before the present. How is it that Roger learned of the Road Poneglyphs and their significance? What if he learned of them on Lodestar island, and set out to search for them?
Roger learned that there was a Road Poneglyph on God Valley and teamed up with Garp to take down Xebec before he could get his hands on it. But after that, Big Mom escaped with that Poneglyph, and Roger later had to steal a rubbing.
Theory by UltraBananaBuddy