In real self-defense situations, the head is a primary target. If you only ever deal with punches to the head in theory and not in practice, you won’t be at all prepared for what it’s like if you actually get punched in the head.
When I first introduce sparring to orange belts, it’s under very controlled circumstances. Students wear 16-oz boxing gloves and mouth guards and they spar at 10% intensity. Only punches and kicks to the shins are allowed. This gives them the opportunity to focus on one aspect of sparring at a time. At this stage, the most important thing to develop is the guard. When students are only allowed to punch, there are less different types of attacks to be ready for, giving them more opportunity to focus on using the guard to protect themselves from attacks.
The other thing students learn at this stage is mentally dealing with being hit in the head and face. Because it’s new to them, they start out flinching quite a bit, making it hard to defend. Also, when they take that first punch in the face, even though it’s a light one that doesn’t cause damage, it still fazes them and puts them on the defensive. After a few sparring sessions though, this reaction lessens as their mind begins to cope with the state of duress that comes from being hit in the face.
As the attacker, by actually punching to the face while sparring, you learn to do it for real. When you only ever spar stopping your attacks just short of the face, your muscle memory develops to do this naturally. A former student of mine who was a 2nd degree black belt in Taekwondo discovered this in a situation when he was forced to defend himself. He tried to punch his would-be attacker in the face and his punch stopped just short of the guy’s face. Fortunately, the attacker took this to be a show of skilled restraint and decided it wasn’t worth it to get in a fight.
While many people don’t like the idea of punching to the face or being punched to the face, it is an important aspect of self-defense. The key to alleviating students’ fears when it comes to this is starting out in very controlled circumstances then upping the ante gradually as their skills improve.