Today, a defensive back gets to bump the wide receiver within 5 yards of the line of scrimmage and that's it. Fans generally like high scoring games. Thus, the rules are manipulated in order to foster a higher scoring game. In the case of the defensive back, tactics which were available to him 15-20 years ago are no longer available. The balance of power moves to the receiver, the receiver catches more passes, and thus we have more points.
Penalties are also to provide protection for players.
A punter, for example, immediately after kicking the ball, is in an awkward position, neither foot on the ground, one leg extended high in the air. If he gets belted in that position, he's going to get hurt. In recent years, rules regarding protection of the quarterback, who is also in a vulnerable position as he's throwing the ball, have come into play. Roughing the Passer Defensive player smacks quarterback after he's thrown the pass or drives the poor sob into the ground for no good reason. You can't tackle or block a player by grabbing a face mask.