TLDW: It's the Law of Unintended Consequences -- The sidewalk shed (aka "scaffolding") is a hack to get around a law requiring building facades be inspected every 5 years. Instead of actually inspecting the building masonry and making any necessary repairs, just install a sidewalk shed and leave it there year after year.
Wendover made a short video about that: https://youtu.be/lphKvTqovHs?t=1m56s
TLDW: It's the Law of Unintended Consequences -- The sidewalk shed (aka "scaffolding") is a hack to get around a law requiring building facades be inspected every 5 years. Instead of actually inspecting the building masonry and making any necessary repairs, just install a sidewalk shed and leave it there year after year.