Suppose an air intake system supplies outside air for process cooling. What happens when the air intake blower motor throws a bearing?
If the intake airflow isn't monitored and alarmed, the lack of cooling air would not be discovered immediately. Only after the process began overheating would anyone suspect something's wrong with the equipment.
You'd probably not discover the motor failure except during scheduled maintenance or while investigating the overheating problem. How many troubleshooting steps (e.g., adjusting the controller) will it take before you even look at the motor?
Replacing the motor restores the airflow, but what have you repaired? There's a defect you didn't fix. It's the lack of instrumentation at several vital points in this system (no vibration monitoring, supply current monitoring, airflow monitoring, etc.).
Due to its location, the motor is vulnerable to fan blade imbalances. Also due to its location, nobody will know when a problem develops. This can easily mean prolonged energy waste followed by failure followed by process downtime.