Interacting with your computer (or computer-driven device) through natural vocalized speech adds a whole new dimension to human-machine interfaces. On the one hand, it adds both convenience and efficiency, avoiding the need to type everything in through a keyboard. In many cases, a voice interface can open up a whole range of uses that previously had been locked out. If you're driving a car or operating heavy machinery, you need both hands to operate them safely—it's just never a good idea to be staring at a keyboard while you type when traveling at 60 miles an hour.
On the other hand, talking to your computer can also take some getting used to. For many of us, it is socially awkward to be talking to your toaster in the middle of a dinner party, or to be dictating out loud your medical problems to your phone on an elevator full of strangers. This may get easier as social norms shift, but some of these scenarios will likely always need other forms of input to help protect our privacy...