Hey Chuck,
No specific reason other than the marketing in saying, never have to buy chlorine again, because Potassium Monopersulfate, "technically", is not chlorine.
You can use really any chemical that is compatible with chlorine, including chlorine. The only one I would recommend avoiding is an algaecide used for yellow algae or mustard algae, and contains sodium bromide. A salt system will react with sodium bromide and cause a problem with maintaining a chlorine residual.
Hope this helps,
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