Broca's Voice is designed to improve communication for patients who have difficulty speaking. There are lots of text-to-speech programs available (thought very few for the iPhone) and all of them share the common problem of delayed speech. The user needs to complete a full word (if not a sentence) before beginning to speak.

Broca's Voice uses sounds (or phonemes) of the English language to communicate. This way, the user can be heard at the onset of speech. 

Plus, in any given sentence there are far fewer sounds than there are letters! This system just makes sense.