How does interpreting secondary and primary sources us look into the past?

History has always been written by those who win wars or win at some other action. Primary information sources may appear to be biased, or one sided and not tell the entire story or chain of events that lead up to a changing event.
Secondary sources of information may be nearly as important as primary because the information.may appear to be less biased toward one side, give another point of view of a story, as well as involving other witnesses to an events.