I don't know whether you have ever worked for a company without vision. With just a plan to survive next week or quarter. I worked for a huge corporate as well as small business and all but one had a vision... well, to change the world. Always but one - and the one was a complete waste of my effort.
You probably know some of the famous speeches, like JFK's request to the senate to fund the Apollo program to conquer space, or Chaplin's speech in great dictator asking for peace. Both are great, inspirational and would make me start a physics degree or let me aim for a Nobel's.
But why sometimes sharing the vision works and sometimes it's just a nice fairy tale?
I believe the vision itself is not what fails. It's the lack of the next steps. If you have just the vision, and no path to get there, expect someone else to fulfill your visions. What I believe every leader needs to show is - at least - the first step.
Whenever you want to share a vision, some future and futuristic goal, you should have an idea about the next few steps. It could be a few numbers to watch, some proof of concept, or just your permanent presence (yes, presence and value, not the presence to watch the number of coffee breaks). Or for the troublemakers, important is to remove the first few obstacles and show that the vision is actually possible.
Without the next steps and without personal effort, the vision and the purpose will not work. It'll be just a nice idea. If you don't know how to achieve your vision, your speech will be just fancy bullshit.