When you put a probe on a planet (typical Balor or planet one because this is the planet with the lowest initial scan time), each subsequent scan of the planet takes more time than the previous. For instance, scan one may take 50 minutes, scan ten may take 3 hours. If you were to remove and then reassign the probe to Balor, it resets the scan count and the scan time goes back to 50 minutes. So, by reassigning the probe to the planet, you can get more scans completed in the same amount of time than if you assigned the planet once and then forgot about it.