They are properly called suppressors. Other than some corner cases (small caliber, low velocity, sealed manual breech or gas sealed revolver), they don’t actually “silence” a firearm any more than a ‘muffler’ makes a car completely silent. You can still hear the shots (and on a selfloader, the action of the weapon) but the sound pressure level is not hearing endangering any more. The “silencer sound” played in Hollywood is not what almost all suppressed firearms sounds like. A .45 ACP pistol firing through your typical suppressor makes a sound like dropping a hardback book off a shelf and you can clearly hear the surprisingly loud sound of the action cycling. It is not at all ‘silent.’
As for legitimate reasons, please see my post upthread.
Hiram Maxim patented it as a “silencer”, though the tacticool crowd has taken to tr term “suppressors”, and pinching the front slide from underneath (to clear the racegun optic that isn’t on their pistol). They also call some soldiers and sailors “operators” these days too. I always laugh at that one, my mom was am operator. Silencer is a legitimate term.