An elevated temporary platform is called a scaffold. The scaffold is used because it is much safer and efficient to have workers and their materials on a platform than scrambling up and down ladders all day.
A scaffold is also the platform on which criminals used to be executed publicly by hanging or beheading. If you watch a lot of swashbuckler movies, you'll have seen depictions of a helpless person being threatened with beheading by a villain on a scaffold in a crowded square. Suddenly, out of the crowd a hero appears and saves the day, after which they ride off into the sunset.