Think of a silencer as more like a muffler on a car. Sure all the real noise is in the engine, but there is still some sound comming out the tailpipe. Even a hollow can on the end will catch and change the tone of the sound coming out the muzzle end (the end in theory you have pointed at your target).
Foam filled or baffled will swallow a bit more of the report.
Is it worth it? Maybe, a good one will cost you $50-100, add weight so your weapon becomes muzzle heavy and length so you'll have issues if you've already got a long gat like a M16 or a FAL it's going to be a hassle, maybe not so much on a MP5 or AKSU-74.
Suppressors on GBB pistols? Why not, you'll have sound bleeding out as the slide recoils, and the muzzle heavy effect is more noticable but if it's the look you're after then go for it.
And last, call them "suppressors".
Airsoft, where nothing is hurt but feelings.