Never having lived in the snow, this concept was new to me. It lead me to this terrible article[1] which says the same thing in each of it's four paragraphs(!), but linked to this article[2] which has a better description of the cultural phenomenon, including some light flamewarring in the comments...
[1]http://www.wbur.org/2015/02/23/space-saver-enforcement [2]http://www.wbur.org/2015/01/28/space-savers-boston