When you punch, you can start at the same place and keep punching. When the opponent gets hit, he will not stay there. He will move to save himself or he will move away automatically after getting hit.
When you practice, that's different. For example, if you practice on a punching pad on the wall, it will not move away. So you can stay there and keep punching. But if you practice punching on a punching bag or kick bag, it will move backward and swing.
In a real life, you cannot expect the attacker to be at the same place, so you have to move while punching. Based on the attacker's body position and move, you can do counter-attack and punch.
When you punch, you can move your body to generate power. Now where you want to hit, that's up to you. You can hit to the ribs or face, solar plexus.
If you are in the rang of attack, you cannot just stay there and do nothing. You have to defend yourself. You have two options. You either move away from the line of attack so that the attacker cannot hit you or you have to defend his attack. It could be punch or kick. Now you are defending yourself aka your body from getting hurt. No matter how the attacker moves, his ultimate target is to hit on your body.
You might get confused seeing his hands movement and start chasing his arms. If you block chasing the attacker's arms, you might end up getting hit by another strike.
If you parry and cover your body, that is enough to defend yourself. Out of fear, if you get freeze or extend your hands too far as a defense, the attacker gets a chance to hit you. And do not move your head away to look another direction. Look at the attacker, you can see his movement. As a result, you can make your move to defend yourself.
Thank you for taking the time to read this post.