This system balances the pH (acidtiy of the water). Between it keeping the pH in check and your salt system keeping the free chlorine in check, that is the lions share of pool chemistry upkeep for most people. And when those things are being maintained consistently, you don't typically need to dump in large amounts of chemicals all at once since things haven't been able to get out of control. As with any pool, you would still want to check for other chemistry levels such as stabilizer, calcium hardness, salinity, etc...
We have more detailed sizing information here, but if your pool was 20,000 gallons, you could consider two 20lb tanks.