Port-knocking mainly mitigates slow distributed-brute-force login attacks, and works best when ports are interleaved with several tripwire black-hole and knock-port-close firewall rules.
Use-cases:
1. helps auto-ban hosts doing port-scans or using online vulnerability scanners
2. helps reduce further ingress for a few minutes as the hostile sees the site is "down". Generally, try to waste as much of a problem users time as possible, as it changes the economics of breaking networked systems.
3. the firewall rule-trigger delay means hostiles have a harder time guessing which action triggered a IP ban. If every login attempt costs 3 days, folks would have to be pretty committed to breaking into a simple website.
4. keeps failed login log noise to a minimum, so spotting actual problems is easier
5. Easier to forensically analyze the remote packet stream when doing a packet dump tap, as only the key user traffic is present
6. buys time to patch vulnerable code when zero day exploits hits other hosts exposed services
7. most administrative ssh password-less key traffic should be tunneled over SSL web services, and thus attackers have a greater challenge figuring out if dynamic service-switching is even active
People that say it isn't a "security policy" are somewhat correct, but are also naive when it comes to the reality of dealing with nuisance web traffic.
Fail2ban is slightly different in that it is for setting up tripwires for failed email logins, and known web-vulnerability scanners etc. Then whispering that IP ban period to the firewall (must override the default config.)
Finally, if the IP address for some application login session changes more than 5 times an hour, one should also whisper a ban to the firewalls. These IP ban rules are often automatically shared between groups to reduce forum spam, VoIP attacks, and problem users. Popular cloud-based VPN/proxies/Tor-exit-nodes run out of unique IPs faster than most assume.
Have a nice day, =3