The Sum of All Parts
Each part of a song imparts a different kind of information. There are no unbreakable laws as to what kind of information must be contained in a given section, but knowing the names, traditional forms, and functions of the different parts of a song can help you to write more effectively and make it easier to communicate with publishers, producers, studio musicians, and other songwriters.
By giving a different kind of musical information in each part of the song, you lessen the chances of a listener becoming bored. By repeating this musical information each time a section comes back around, you give a blueprint of sorts that helps guide the listener through the song without becoming lost or confused.
Although some sections of a song may give new lyrical information each time they occur, the way in which the information is presented, as well as the rhyme scheme, meter, and type of information, is the same every time the section repeats.
If your song had no recognizable form to the melody, meter, rhyme scheme, or lyric structure, it would be hard to follow and nearly impossible to sing along with. Even if you're one of those songwriters who delights in breaking the rules, it helps to know them first, so you can make sure that you're breaking, bending, or twisting a given rule in exactly the right way to make your publisher have a seizure. Read on: There might be rules you didn't know about, waiting to be broken.