Big Smoke wants Carl with him to collect his cousin, and the collection turns out to be an ambush that leaves one Vago running. Everything the mission actually consists of happens after that: a foot chase through Los Santos after a man who does not want to be caught.
It is a short mission and an early one, and the reason it matters is what Big Smoke is doing in it. Read once, it is a favour for a friend. Read after the story has finished, it is one of a run of errands whose purpose is not the one Carl was given.