An optimal (slower) respiratory rate provides a) a better long term oxygen supply for the lungs and thus the body and
b) signals to the brain that there is currently no acute dangerous situation and therefore no need to fight or flight - and can thus help the body to relax and to reach the parasympathetic mode, which is important and responsible for healing and regeneration and therefore the immune system.
Try it yourself, relax and exercise at the same time – HeartBreath(e)