There are actually a couple of reasons why you would use a bino harness, the main one is that they securely and comfortably attach your binoculars to your body, freeing up both your hands when you are not actually using them. Your regular binocular strap can be sufficient, but they don't usually prevent your binoculars from swinging about and are less secure than a harness that makes it almost impossible to drop your optics.
1. Unclasp two buckles of the harness strap. 2. Thread two short harness straps through two eyelets of your binoculars/camera (as steps shown in picture). 3. Put on the main part of the harness strap and adjust it to fit your body. 4. Attach your binoculars/camera to the harness strap by clasping the quick-release clips. 5. Lengthen or shorten the small sliders near the quick-release clips to lower or raise your binoculars/camera (be careful to get each one the same length otherwise the binoculars/camera will not sit straight).
The triangular shape that the harness makes between your optics, your face and where it attaches to your body, along with the elastic attachments that pull the binoculars slightly in towards your eyes makes for a very stable shake free view and certainly more so than without.
The harness make for a very secure way of carrying your binoculars, with no chance of you accidently dropping them. Also unlike a neck strap that leaves your binoculars to swing about, a harness keeps your expensive instrument close to your chest and out of harms way.
Four Directions to Disperse the weight of the instrument. A good harness for binoculars is also the most comfortable way to carry your instrument which just simply adds to the enjoyment of whatever it is that you are doing.