"Open world" can mean more than one thing. In this case, it means that many quests are not required and can be skipped, and many can be done in any order you please, or at any point in the game, and that Lisa can wander freely around the available maps, triggering any available events. But that doesn't mean that all quests are optional, or that all of them can be done in any order. This game also has a definite story, which means that some quests must be completed, and some must be completed in a certain order. There's no other way for a well defined, in-depth story to be told.
As for the trip with Jo, it's obviously not pointless. That's such a ridiculous thing to say that I have to imagine that you actually facepalmed yourself when you typed it. That sequence introduces the first three paths for Lisa's relationship with Danny (Cheating, Faithful, and Sharing), it introduces three new characters into the game, and it gives us 2.5 sex scenes (I'm counting Jo's blowjob scene as half of a sex scene.).
The relationship with Danny will become optional later in the game. For now, it's part of the story.