Debug a possible situation or even something that happened in the past. Take for example these question from ServerFault: "DNAT:ed packets disappearing"* [1] and "Openvpn, forwarding slow since reboot" [2].

*Sorry for the self promotion, but I don't have any better examples right now.

[1]: http://serverfault.com/questions/320181/dnated-packets-disap...

[2]: http://serverfault.com/questions/542381/openvpn-forwarding-s...