Do you have examples where seeing the content of packets is possible or significanty easier than also being able to supressing the delivery of packets or modifying the packets?
Wireless networks are a good example. Many switches provide monitoring capabilities and fibers are not that hard to sniff. So someone with physical access can look at the traffic with relative ease. However, in order to block selected packets the attacker needs to redirect traffic or install equipment in the middle. I suppose that's doable on wireless lans but not so much when tapping into existing monitoring capabilities.
In this case, attackers block all the packets and relay almost all the packets (except for a few selected packets) by themselves copying everything include MAC addresses, which hides the attack from monitoring, which is easy both on wireless and wired lans.