isolated vs. non-isolated DC/DC converter in PoE
You need an isolated converter when the device should be floating instead of referenced to the ground of the PoE injector.

Since the injector may be in a different room or building, and thus may have a different ground than the one the device is placed in, no exposed parts may be connected to the circuit ground reference if you are not using an isolated converter.

This is similar to Grounding a POE-powered device.