the short, short, short version is that in order to make accurate maps, you have to have a system of reference points (AKA a datum) that 'attach' the symbology on the map to the actual features on the earth's surface. This directly affects your ability to make maps out of it. satellite imagery is completely unprojected-it's functionally in a geographical coordinate system (GCS) rather than a projected coordinate system (PCS), so it's only in 2 dimensions instead of 3, and there is no datum.
