Some people look really tough, but they go down from a tap on the chin. Whereas, trying to knock a guy out by punching his skull can take a while, unless you hit really hard. Head's like a helmet. Not a good target, unless you can already break patio blocks with your fists. I've knocked people out by punching their skull without hurting my hands, but it takes a while to get your fists tough enough for it.
Some people use the jab in a light way, like a fly swatter.
Sparring - The quality of your sparring partners will influence your skill level. Highly skilled fighters do not need to go full contact all the time to get a lot from the exchange. I want to be able to remember my wife's name when I'm 60. Moreover, you can't explore new combinations and options if there's too big a price to pay. When somebody is out there trying to knock your block off all the time, you'll tend to fall back on just surviving instead of consciously enforcing actions that are intelligent if not yet reflexive.