This is actually the biggest reason I marched on October 26. Given the impending United Nations vote, this particular juncture in time was an important one to remind the world that the US is divided on the issue.
Congressfolks don't care about US opinion as a whole-- they care about their constitutes so local organizing matters more for Congressional votes. But for influencing world opinion, door-knocking won't have much effect, so that is probably the best justification for a mass march.
-- Nathan Newman