You can statements about people you know in sign that they cannot understand and likely never will. You can be truthful and blunt without getting your ass kicked.
You can have a 'private' conversation in public places as long as there are no other fluent signers around.
It can be way to teach others about Deaf culture if they see you signing in public.
sometimes you just cant describe something in words and you need a visual aid.
Signing is great for all these reasons and more.
My favorite use for sign is with toddlers (Hearing, I have yet to meet a Deaf or Hard of Hearing toddler), especially at that stage when they can't quite vocalize, but can move their arms just enough to begin to sign. They are so very cute to watch when they sign. Just today I asked a boy if he wanted "more" cheerios "please". He didn't get the "please" part down, but when I said can you say "thank you", he turned around to look at me and signed it while he vocalized it, and smiled. Soooooooooooooo cute.