Re: Talk to a computer? why would I do that?
It can be useful in hands-free situations when you really need to be concentrating on something else. As in your car analogy. But that's only really viable if you don't have to think hard about how and when to issue the voice command.
If it's not natural language, without special pronunciation, it becomes a distraction in its own right.
If there's a delay between pushing the button and being able to issue the voice command, that's another distraction. Not desireable while driving.
At the moment it still seems too gimmicky, too clunky to be useful. Most times it's often easier and quicker to grab the remote / pick up the phone / flick the lightswitch.
Case in point: step lad had hooked up Alexa to a "smart" lightbulb. Enters his bedroom annoucing "Hey Alexa! Turn on the lights.", whilst passing the lightswitch right by the door. Ridiculously pointless.