Great songs don’t require rhymes. Great poetry doesn’t, either.
So why do so many have them? From Shakespearean sonnets to the Beatles to Stephen Sondheim, rhymes have a massively important place in the creation of both song and poem. But none of it is required. The art form includes them because humans chose to include them.
When you realize that rhymes and rhyme schemes are things that were invented, that they were developed over time as an artistic choice, then it becomes urgent to ask, “Why?”
I think there are two reasons. One for the performer, one for the audience.
First, for the performer – I think of Homer, a blind Greek poet reciting thousands of lines of verse in his tales of The Iliad and The Odyssey. I am convinced that using rhyme and meter was essential to Homer’s memorization process. Holding onto text is easier when it rhymes.
Second, for the listener – it helps comprehension immensely to have a rhyme scheme. Our subconscious minds can follow along, and often make great predictions about future rhymes. Rhymed words can retroactively help us make sense of words that came before. And the rhymes can have a catchiness to them that helps with connection to new music.