Bob Fosse said that most songs fall into two categories: the "I Want" Song and the "I Am" Song. The "I Am" Song is, simply put, a song which establishes a character's personality, role in the plot, and/or motivations right away. It can be more effective than setting up a character by dialogue, and it's almost always more fun.
Despite the name, the song doesn't have to be sung by the characters [...] [tvtropes.org]