A barge recently collided with a bridge support causing it to collapse. If convenience stores and other locations use poles to prevent patrons turning them into drive-ins, why not use them to protect bridges?
Barges(or ships) don't often collide with bridges. But when they do, it can create a very large mess. Using poles to protect bridge supports is relatively cheap insurnace. A secondary use might be to allow fisherman to use them as anchoring or tie-off points.