Through ornamentation a singer gives dramatic structure to a song, and this requires an understanding of the 'story':

Different lines in a song are different, sung differently to other lines in a song, some verses tells you of the tragedy and other verses tells, why did it happen and they vary, like that the same way you sing any version of a song. There's an old saying that you start a song slowly, you build up a climax and come down gently. And in folk music, there is no beat, it's just a pulse (Heaney 78-15.1).

From here.