Black plastic shade balls have been used for all sorts of purposes over the years. They've been deployed to keep birds out of water near airport runways, control vapors in industrial ammonia tanks, or stop water from evaporating at petroleum operations.
The city of Los Angeles has realized that shade balls can also be useful for protecting drinking water from contaminants. Back in 2007, the LA Department of Water and Power discovered its reservoirs contained a fair amount of bromide and chlorine. The city decided it would shield some of its reservoirs from the sun. And shade balls were a low-cost option.