There may be situations where they are erosion control, but there are definitely many places where they are deliberately anti-homeless (or refugee) measures, in

There may be situations where they are erosion control, but there are definitely many places where they are deliberately anti-homeless (or refugee) measures, including the ones in the article. See coverage here (for example) https://www.dailysabah.com/europe/2017/02/20/paris-authorities-place-boulders-under-bridge-to-prevent-refugees-from-sheltering https://www.dailysabah.com/europe/2017/02/20/paris-authoriti...