“Where there is no vision the people perish.”
OK, a bit melodramatic for talking about RPGs. Lets come down to earth.
I think most of us have had that experience of imaging what game play might look or feel like as we consider a new game, or prepare a campaign. Maybe we were moved by the artwork and have had our imaginations sparked by possibilities. Having that clear vision of what kind of experience we are looking for can be very helpful, but it can also be fraught with peril.
As long as we don’t over visualize or prematurely imagine the events of play, I think that we are able to responsibility adhere to the vision that the group has settled on in “session zero”
If we don’t have a clear vision and set of expectations, then we can expect a lot of “faffing about” without direction and meaning. But if we have taken the time to bring our collective vision for the experience into sharp focus, we can expect to have plenty of rewarding vistas to gaze upon, all of which fit our expectations from our session zero
IvanMike1968 
I am not in the camp of the faffers and prefer there to be minimal faffing if at all possible. One thing that I am noticing in the vocal “We don’t do a Session 0” camp is that they are operating on shared, learned, and socially reinforced assumptions of a particular culture of play (whatever the particulars are for each of these folk) – and so…. benefit in the same way those who are strangers to each other and the new game benefit from a session 0.
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