Understanding ISO, shutter speed and F stop

In a recent forum entry a photo of the moon taken when it was close to the earth included the metadata showing a relatively high iso, lens stopped to f16 and a fast shutter speed (image taken on a tripod). I understand the aperture setting to permit a deeper depth of field. However, I don’t understand why the iso was set so high when the shutter speed could have been set much slower to obtain the same EV. Assuming motion blur and depth of field can be controlled, wouldn’t one obtain better resolution at iso 100 rather than 1600 by using a slower shutter? What is the benefit of not using the lowest iso possible given that the desired depth of field and motion blur or camera shake can be controlled while properly metering (using a tripod for example) by simply slowing the shutter?