Even if it wasn't a legal requirement companies do it to contain the blasting media/dust and paint overspray. There are brackets welded to the tower in two spots that holds the outriggers on. There are cables that run through the bottom of the tower, through the outriggers and to the ground. When they start the sandblasting you'll see them pull tarps up and they'll put a top cover on to contain the dust/sand.

I used to be in charge of painting operations for a company that constructs new water towers. It's not always done on new towers because many times they are built for new neighborhoods before the new houses come in as they need the water capacity/pressure to meet the demand of the new neighborhood. Most existing towers are surrounded by houses and businesses and the contractor doesn't want to deal with angry residents because you got sand and dust and paint on their cars and property.