[sophacles]

Subtitles (like sms or twitter) don't convey tone very well, so in a classic voice over just having the subtitles makes it hard to convey things like sarcasm or innuendo or emphasis or emotion. Sign language has ways to convey all of this.

Think about this sentence:

That's cool.

It's one of those sentences that exists only to convey a tone. Depending on tone it can mean:

* that is in fact cool

* that is in fact NOT cool

* i acknowledge that you have said something

* i agree to your suggestion

and plenty more in contexts where you only know which it means because you can hear the tone of the person.