The characters in the story are all rearing and ready to go! They need something to keep their spirits up, something to heighten the moment: A MUSICAL NUMBER! This is the number that everyone sings when they set off on their adventure. A "let's get ready and go" kind of number. Something jaunty and fun.
Often crosses over with the "I Want" Song, and may be an Ear Worm. Compare Wanderlust Song. [tvtropes.org]