The video explains that the evaporation reduction was a side effect. The real reason is to reduce the level of cancerous Bromide down to safe levels, which could only reasonably be achieved by blocking sunlight from the water.

They tried a bunch of ideas for blocking the sun and settled on the balls as the best compromise between cost, effectiveness, and safety.

There are a couple of other benefits. They need far less chlorine to manage algae growth and it discourages waterfowl from taking up residence in the pool, also increasing water quality. They expect the balls to pay for about half of their cost over an expected 10 year lifetime from the savings in chlorine and evaporation.

I can tell you didn't watch the video