In dark environments, the people often complain that their pictures aren't sharp. This happens because the shutter speed isn't fast enough, particularly for a subject that moves quickly such as this bobcat I photographed today during my wildlife babies workshop. Of the three crucial factors that have to be decided every picture -- shutter speed, lens aperture, and ISO-- it is the shutter speed that is the most important. If the subject isn't sharp, it doesn't matter how much deph of field you have, and it similarly doesn't matter how much digital noise you don't have. Therefore, you have to choose a high enough ISO to give you a faster enough shutter speed for a sharp picture. You can minimize the noise later in post-processing. In addition, the lens aperture has to be large enough (and often it has to be wide open) to let enough light into the camera for that fast shutter.
If you are shooting landscapes or architecture, the shutter speed doesn't matter because nothing is moving. With a tripod, you can use any speed and still get a sharp picture. But with wildlife, birds, sports, etc., the shutter is the first consideration.
This picture of the bobcat in a tree was taken when it was raining, and inside the forest the conditions were quite dark. I used 1600 ISO to enable to me use a shutter of 1/320. The lens aperture was f/4.5, and I used a 70-200mm along with a 1.4x teleconverter.