Punting. Giving the ball to the opponent. Giving away scoring opportunities.
Here is a good way to sum it up:
Here’s a thought exercise for you. Imagine that for decades no one ever thought of the punt. Teams knew nothing else than to run or pass on 4th down. And then one day it’s invented. Some guy comes up to a coach and says, “Kick the ball on every 4th down and the other team gets possession 37 yards further down the field.” The coach would think he was crazy: “Wait, you want me to give up one quarter of my opportunities for a first down on every series…just for 35 yards of field position? Do you realize how much that’s going to kill our chances of scoring?”
Every single serious study of 4th-down decisions has found that, in most situations, teams would be better off by going for the conversion attempt rather than kicking.