I've never really gone for the whole vision "stuff" as the process often feels a bit unfocused and the outcome too non-specific to be of much use. However, today something clicked for me which made sense. I realised the purpose of the vision is to provide a map for the swampy area of our projects.
What is the swamp? It is where we are thinking about the future (time beyond immediate knowledge) and where this time intersects with large scale system change (lots of people, lots of stuff going on..).
If I have the desire to change a process which is mostly down to me and something I can do now, this feels a fairly certain and concrete process. It gets a bit more complicated if I think further into a future and also involve a wider group of people - team, department, organisation, community etc. At some point the amount of variables involved increase beyond the point of managing the process and even conceptually controlling the desired outcome. here I enter the swamp. A vision, therefore is the map I carry with me which has an outline, a representation of what this future holds for me and all the people also impacted by it. This helps me make sense not only of the future I am part of designing, but also of the present; it enables me to contribute to the uncertain future through my more certain actions right now.