Monitoring doesn't prevent hardware from failing. It gives us warning, most of the time, and warning is what turns a replacement into a scheduled task instead of an outage.
It doesn't catch everything. A power supply can die without notice and a fiber cut doesn't send an alert first. Some failures are instant, and no agent changes that.
It also isn't security on its own. Patching closes known holes, which matters a great deal, but it works alongsideendpoint monitoring,email security,tested backups, and multi-factor authentication. Any one of those alone leaves a gap.
What monitoring reliably does is shrink the number of problems you find out about from an employee, and it gives us enough lead time to have real options on the rest.