I just don't get what one needs a fast car for when you can't even travel at a speed adequate for that car on public roads.

Because a car that can do 140mph can do 70mph with much less effort than a car that can only do 80mph. This results in a nicer drive, a safer car (because it's designed to go that fast -- brakes, etc.), and less wear on the car. It's probably also better for the environment.

Once you've got a car that can do 200mph, that's overkill for this argument, but who cares? Take it out on the track (e.g., a drive-what-you-brought day), and have fun.