ok if you own one tank, if one of your fish get sick or you make the mistake of mixing aggressive fish that are destroying your other fish, your screwed. also you can't do any quarantines with only 1 tank so your always playing russian roulette with your fish. alot of people get mislead to think fish die from bad luck if you talk to people who owned fish they couldn't handle you always hear the same story "oh yea i had a ____ once! but it died. oh how unfortunate." without even knowing they had more power over the situation than they think.
so yes. real fish keepers have at least 1 spare tank as its a necessity to the hobby for many different uses including quarantines, breeding, hospital, "prison tank"/separation, cycling, experimenting and testing, and storage.