The store bought water is usually reverse osmosis water and should be free of chemicals, minerals and chlorine/ chloramine. If you keep soft water fishes and live in an area with very hard water, then it would be better for the fish.Store bought dechlorinated water is definitely not as good for your fish as say Seachem Prime.
It’s so stupid that people buy that “Betta water” (I believe it’s a 1g bottle) for $8 a bottle, when I can buy and treat over 1,000g for $12.