Hi, yalgaar.
Here’s my take on it – the slow breathing gets your body to relax and access the ‘rest and digest’ processes which are necessary to healing and re-building. It’s a part of life that’s been neglected in the United States recently (and it leaks from here to other nations), so it’s one of the differences between a healthy life and a stressed out life that yoga can teach.
If a particular person (in a particular body) needs to reduce weight, losing that weight will also reduce stressors on the body. Cardio is great for that (if the person is healthy enough to start there.) It’s a different technique (and different breathing) because the body is coming from a different way of being not healthy.
Both can contribute to the same goal – a healthy mind in a healthy body.