One of the signature features of Phineas and Ferb is the catchy music that's played in every episode. It's almost like it was always supposed to be that way. Except it wasn't.
The producers never actually intended for there to be a song in every episode. The only episode that was supposed to have a song in it was Flop Starz in which Phineas would write, record and perform the catchy song "Gitchee Gitchee Goo", become a one hit wonder, then retire.
Disney Channel executives saw an early cut of the episode and absolutely loved the song. They requested that each episode, even the ones already produced, have a song in them. And the creators obliged. Music became a huge part of the show, inspiring theme park shows and soundtrack albums. But it all started with the greatest song of all- Gitchee Gitchee Goo.