Yes they are. They remove nitrates in the water which can irritate fish. With a big tank, say 180L, a 20L change should be done at least once a week depending on how dirty your fish are, how many you have etc. You wouldn't want to be swimming in your own piss and shit, put it that way
If you want to save costs on water treatment products, buy yourself some of this Seachem Prime and these 1ml syringe
5ml treats 200L of water compared to 25ml that other brands use. I used to do 20L changes, only took around 0.2ml to treat it.