As others have mentioned, the primary use of TTS on Android is for navigation, but it can be used for speech output for anything. Google Now uses it for answering your questions (similar to Siri on the iPhone):
Me: "Google, what is the answer to life, the universe, and everything?"
Google (using TTS): "The answer to life, the universe, and everything is 42."
Also, as others have mentioned, the TTS Google included with Gingerbread and earlier sounded like a robotic imitation of speech. SVOX was pretty much the leader on Android and was a "required" purchase if you were going to use Google Navigation (more natural sounding). Ivona came out with a free beta on Android about a year and a half ago, and the voices are as good, and in many cases, better than SVOX. The built in TTS included with Ice Cream Sandwich and above is much more natural and I've not felt the need to load a third party TTS, but I'm still a fan of Ivona.
