The manufacturers themselves boast needing less chlorine with any of the secondary sanitizing systems, except, not a one of them block out the sun, which consumes up to 4ppm of FC a day this time of year. (Maybe even more in the uber hot places). They all claim you'll need less daily FC than the sun consumes, with your average pool owner not knowing the difference, or why they're consuming chlorine in the first place. They just see 'need less chlorine' and jump on it.
These secondary sanitizing units are all about algae control after the fact, and they help some with that, but nobody questions the industry advice which all but ensures you'll benefit from a device to help control your algae problem. Notice I said help and not fix. Algae grows exponentially and nobody to date has a device that can overcome that with the wave of a magic wand.
Replace the daily FC loss from the sun, with some wiggle room for whatever else happens such as a get together or a particularly high UV day, and you'll never have algae to need further help with. It's literally as simple as that.