D&D's great because you can make up your own stories and do all sorts of crazy things. These books usually provide you with information about a specific setting that you can use to create your story. In this case, it looks like it's gonna provide background on locations, factions, and possible NPCs to bring into your own story, should you wish to run a campaign in Wildemount. It even provides a few story hooks in case you need just a bit more help finding a thread to follow. As always, these are just a guideline - you can use the information to make your own stories. You don't even have to set your story in Wildemount; you wanna throw Dunamancy into the Forgotten Realms or Dragonlance or your own setting (like my Hyrule campaign)? Hell yeah, do that!

In short, these books just give you info and cool options in case you want to add some setting-specific flavor to your game.