I'm trying to track down a copy of the book that the story is in. I believe it's from "Embraced by the Light" by Betty Eadie.
Basically, she had a near-death experience and she was shown not only her mission, but the missions of some others as well. One man that she was shown was a bum on the street whose only mission was to be in a certain place at a certain time and smile at a specific lawyer. I want get the actual wording from the book, and maybe it will be more clear.
Anyhow, it amounts to we each have something to do, and even the smallest act (a smile, in the above case) is the biggest act for it starts a chain reaction of other events that we may not even be aware of. Sometimes we are where we are because we need to be there for something else to happen.