I agree it is wrong to believe that port knocking improves security, but it undoubtedly improves reliability. For example i just spend ~50 minutes attempting to get in line to ssh opnsense, that refused browser logins with "CSRF check failed." error. Turned somebody managed to find the ssh port and over jealously attempted to bruteforce it using large bot net, filling all free space with filter.log's. Unfortunately ssh has no distinct answer that can inform bots that no password logins are allowed and non-standard port when found triggers much more intense attacks, as bot writers most likely consider such servers more interesting. In cases like this what turned to be quite successful DDoS would be impossible if i had port knocking set.