Have a think about this. If your foot worked the same way in real life as it does on the treadmill then the Earth would have to move under your feet would it not? Common sense and a little bit of physics suggest that the Earth is more ‘Massive’ than your body so it does not move underneath foot. So then why look at your running gait on a moving platform.
I think the main reason would be to have a look at how your foot performs in different types of shoes compared to one another in a controlled environment. This also allows you to video the persons feet from behind, which is useful to help you sell the trainers right!? Don’t be fooled into buying running shoes based off what your foot does on a treadmill, your foot works differently. makes sure you get someone to look at what your foot does on the ground, which doesn’t move underneath you.