I am sure someone will have more experience than I but here is my two cents. I understood the two tension knobs to aid in thicker threads. If you need a lot of tension and only have one point you have to really tighten it down like pinching the thread. This can damage the thread, so having two allows you to get more tension with less stress on the thread.

You are right, you have two tensions....the one you are talking about is only for the situation when you are sewing normal and there is a bulk of leather or a cross section....so in this case to sew over the short double layer of leather you can push the second tension (the looser one) to sew over the bulky section...otherwise it could happen that your thread will break due to double or triple layer of material in a cross section...I hope I could explain it to understand...