I love to turn off my iPhone display when my iPhone is doing something that doesn't require my fingers to input anything. First of all, because it saves battery; and second, because then you can't accidentally mess something up by accidentally pressing something. For most tasks (e.g. playing music), all that is required is to press the button on the top of the iPhone. The iPhone keeps on doing it's thing (like playing music) and the battery isn't wasted. This would also be nice when you're talking on the phone, especially if you're using a head-set. However, for talking on the phone, if you press the top button, it will end the conversation. That being said, you can still turn off the display. It just takes a couple more steps.
0. Phone call is already happening.
1. Press iPhone home button (the button on the face of the phone). This takes you to the home-screen and leaves a bar at the top of the display to indicate that there is a phone call happening.
2. Press the iPhone sleep button (the button on the top of the phone which usually turns the display off). This will lock the phone, but the phone call is still happening.
3. Press the iPhone sleep button again. This turns off the display, but the phone call is, as before, still happening.
4. If you want to end the call, mute yourself, etc, press the home button or the sleep button and you will go back to the lock screen.
5. Talk away.
