If you have a [metro 4n], and you don’t quantize it, then it will output a bang every quarter note starting from the moment the metro is turned on. But this will not necessarily be on the beat, because you might have turned on the metro in the middle of the beat.
If you want to make sure the metro is on the beat, then you use quantize. It will send the first bang at the next quantize point, and then every quarter note after that.
If you use timepoints to start a metro on the beat, then you probably don’t need to worry about this. If you let the user start and stop metros, and you want to keep everything in sync, you should use quantize.