In a pool with very low CYA these products are useless, chlorine alone is just as good and much easier to use.
These products will not change the CYA level up or down. They do add a little sulfate to the water, but that isn't normally an issue.
It is the ammonia, which combines with chlorine to form monochloramine, that is the active ingredient. The sulfate and EDTA are irrelevant. The effect goes away as soon as you add the second dose of chlorine, breaking down the monochloramine, well before weeks have gone by. CYA remains high the entire time, it is simply that CYA has no effect on monochloramine, which kills algae regardless of CYA just as well as chlorine when it isn't being restrained by CYA.
Yes, the high CYA level is a problem, and remains a problem, before and after the use of these products. They sell well because many people don't know that, have no idea what their CYA level is and simply notice that chlorine isn't working like it "should".
Correct, appropriate CYA levels make these products completely irrelevant.
One additional thing to be aware of, there are two very similar product lines that actually work in completely different ways and are both only useful when CYA is really high and you can't lower it. Green to Clean, Yellow Out, and several others work using ammonia. The other family, The Yellow Stuff, Yellow Klear, Yellow Treat, and others, work using sodium bromide.
The sodium bromide is activated to bromine with chlorine which again kills algae without being affected by CYA levels. However these products are more troublesome because the bromine turns back into bromide which remains in the water, effectively transforming the pool into a bromine/bromide pool for years to come. Bromine breaks down from sunlight more quickly than chlorine with CYA and there isn't anything like CYA that you can use to compensate, so your chlorine demand goes up more or less permanently.