It's incredibly easy to get caught up in the busy-ness of getting stuff done to the point where you lose focus on what the intended outcome is. A compelling vision provides the answer to "why" in the midst that busy-ness when things may be tough or challenging and you think to yourself "Why am I doing this?"
A lot of us ask that question to ourselves a lot of the time. We may not even take the time to ponder the question. However is that question goes unanswered our subconcious will continue to work on answering it. The less clear the "why" the more likely effort will become diluted and obstacles become increasingly insurmountable. Where teams are involved people can become increasingly disenfranchised and the balance can move from the collective vision towards personal expectation.
The role of a leader is to sufficiently envision his/her team to the point where that "why" is never asked. At least the leader needs to do such a good job of painting the vision, and continually re-affirming it, that becomes indelibly printed on the soul of that team. Then anyone with the slightest thought as to "why" they're doing something will be able to answer that question with what they can see.
Ken Rudge