Two reasons. First, nicotine and the other addictive alkaloids in tobacco are transdermally absorbed. The more skin they are in contact with, the more your body absorbs. So slow exhales and nasal exhales expose more flesh to the smoke for a longer period, drawing more of the drugs smokers are actually after into your system. Same thing with cannabis, a slower exhale may extract more cannabinoids from the smoke.
Second reason is the addiction to smoking, and to anything else actually, has a large visual component. Seeing a large cloud of smoke feels more satisfying. Ask any smoker, smoking in total darkness is surprisingly unsatisfying, and any stoner will tell you that a huge exhale just feels like it does more for your high, even if it doesn't.