Folk songs are an important historical source in that they can reveal the feelings, opinions and social values of people at the time as well as their view of historical events. Look at the number of songs about sailors being inconstant lovers or whose lovers are only interested in their money.

Irrespective of the "winner writes the history", views of what is historical truth also change over time so the songs can also reveal something of how people at a particular time thought of their own history. "Flower of Scotland" is a good example of the Scottish mythologising of their own history (and before anyone starts - I am part Scots and yes the English, Irish and Welsh, and no doubt Americans, are just as bad) and reveals more about the views at the time of its writing than any historical event.