Attacks often happen at inconvenient times. Criminal hacking follows the academic year. Smart criminals attack banking systems Friday afternoon, after they've closed for the weekend. In 1973, the Arab countries attacked Israel during Yom Kippur, the holiest Jewish holiday of the year. If someone were going to launch a serious attack against a system, he would pick an equally inconvenient time.
Vigilance means immediacy.
In any aspect of security, timeliness is next to godliness. It's much more useful to detect an attack in progress than a week, or even an hour, after it happened. It's far better to upgrade your systems in response to a vulnerability now, and not next month. Sometimes reacting late is no better than not reacting at all. Vigilance also means preparedness.