Sign language is used for communication between people where normal speech is impossible(perhaps,say,because you're deaf,or perhaps,say,because your girlfriend doesn't like your oral skills and if she tells you out loud,her mom will want to know what's up with the homework).
Now,a cousin of my mother's went deaf when he wa around ten.They could only afford special school for a very short period,so much so that he never learnt any formal sign language.
HOWEVER,him and his mom(grandma's sister)devised some sign language of their own.She understands everything he says.He understand her too.But the rest of us are left standing in the lights.Unless it's something very basic,like asking him to pass you the salt on the table,which you do by(d'oh!!) pointing at the salt and beckoning it towards you,there's nothing much you can tell the guy,or the guy you.He'll try to tell you stuff about fights(he makes boxing movements or machete cutting movements),about music(he'll strum an air guitar),but you'll never understand what it is he's trying to tell you.
Now,if he'd learnt formal sign language,and if we'd learned it too,we'd communicate perfectly.He'd be able to "talk" to anyone who understands sign language.But since he hasn't,and since his sign language isn't any formal language known,the status quo remains.
That's how I see theory.
Just as learning sign language wouldn't determine what this guy would want to tell me,music theory doesn't determine what I want to say.It simply enables me to say what I want to say.Instead of me walking to town,I take the bus to town.I know where I want to go.I'm just making the process easier.Instead of exploring the grassland on foot,I take a jeep.
Now,I hope that makes some sense.
