The onboard darks can do a better job than the standard dark subtraction proceedure (not always though) as that dark is optimised for the temperature of operation(though there is some non-random noise that it doesn't take into consideration). However it is time consuming. If your capture time frame is limited to an hour due to a front bringing cloud with it you want to spend your time taking your valuable light (signal) shots and then take your darks when your finished. If time is not a worry then you can take your darks using incamera dark subtraction. This is one of the benefits of active cooling in cameras. You can take your lights at a given temperature and then take your darks whenever you want to by simply cooling the camera to the desired temperature. ( well that's the theory anyway
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