When there's under like 30 seconds or so and the only thing that will beat you is a hail mary, you might as well just have extra defenders on the field to guard against the hail mary and eat up clock and then just accept the 5 measly, irrelevant yards that you will be penalized. Hell, don't stop at 12 guys, the penalty is the same regardless of how many extra guys you have on the field. You seriously literally might as well have all 45 guys that dressed on the field. Same with on the goalline. If you're up 9 or more with like 30 seconds left and the other team has it on the 1 yard line. If you play them straight up, they could get a TD and then have time to maybe recover an onside kick with a few plays to get in FG range. But if you just have tons of extra guys on the field to stop a TD, you keep conceding the 5 inch half the distance to the goal penalty while not allowing a TD until the last play of regulation and then you win.