Musical 101 explains: "The Main "I Want" Song comes early in the first act, with one or more of the main characters singing about the key motivating desire that will propel everyone (including the audience) through the remainder of the show.
Bob Fosse said there were only three types of show songs from a director's point of view there were only: "I Am" songs - a song that explains a character/situation, "I Want" songs - desire/motivations, and New songs - A song that doesn't fit the other categories [REF].