Note that this is not just for reading ebooks, but is essential for 'accessibility' of the device as well. It can probably help with learning a new language also (though you might wind up with some strange inflections!). Probably most people use it for GPS navigation, though Fire isn't able to do that.
I like to have TTS capability with the reading apps I use. And it is fun to have a variety of voices to choose from. ePub3/CSS has some markup for controlling aspects of speech synthesis 'such as pronunciation, prosody and voice characteristics'. Maybe Amazon can borrow some of that. For example, this would let the narrator's voice to be different than words spoken by various characters.