Most open world games are a bit of a chore, I find. At first you’re super excited to see what’s happening outside the main missions, but after an hour or two you discover that it’s mostly just following waypoints to collectibles secreted in otherwise featureless landscapes, irritating fetch quests that make no sense in the context of the world, and repetitive actions clearly designed by a totally separate team to the one that created other elements of the game. None of it seems necessary.
CD Projekt RED hasn’t made an open-world game before, and I think that’s why it’s done such a good job this time. No, really: the quests you encounter out in the wilds are generally written like traditional RPG sidequest lines. They’re often triggered by events in the main story, and lead naturally off those events. For example, in the course of the main story you meet up with Keira Metz, who accompanies you on a quest to try and find out where Ciri has gone, because she’s has business dealings with someone who knows Ciri.
At the end of this mission, she asks you to help her retrieve an item from the dungeon you’re just about to exit, which was promised to her by her business associate. See? It makes total sense to do it then and there. As a result of this quest, Keira asks you to come visit her sometime, and if you do so you can elect to embark on a short sidequest series.