Longstanding practice to catch lower level people as a means to get to the top. They don’t have to be directly connected to the primary objective. IF there was collusion these people will be used to gather evidence. While this is political as hell Mueller is following standard prosecutor steps. Don’t read too much into the noise around him until he has enough (if he ever does) to make his move. If he finds enough it will likely be overwhelming. Finally as I’ve said before if he doesn’t find evidence of collusion at the top I hope he doesn’t try to take obstruction as a means of saving face.