What is a Power on Reset IC?
Power-on detectors (also: brownout detectors) check whether the supply voltage has been stable within a certain range over a certain amount of time and let go of the reset line only when that happened.

The problem with unstable power is complex ICs fail partly, so while e.g. an internal EEPROM holding calibration data may only return garbage, the CPU already or still runs with that, now garbage, data.

Whether you need something like this depends on how reliable your device needs to be. If it's interactive so the user can check for unintended behaviour, you probably don't need it. If the device is expected to run unattended, you certainly need such a detector.

Some µCs have this function already built-in.