Chlorine and chloramine are disinfectants added to tap water that are harmless to us but toxic to fish and to your beneficial bacteria, so all tap water must be treated with dechlorinator before it goes in the tank.
Chlorine gasses off if water is left to stand, but chloramine (chlorine bonded to ammonia) does not — and many water suppliers now use it, which is why a bottle of dechlorinator is essential rather than optional.
Skipping dechlorinator during a water change can wipe out your filter bacteria and trigger a mini-cycle, so it is a step to never forget.