How are Homerian epics, or the long oral histories and ballads of the Yugoslavians, the Gola, or the ancient Hebrews, remembered? Psychologists Wanda Wallace and David Rubin, among others believe that the mutually reinforcing, multiple constraints of songs are crucially what keeps oral traditions stable over time. In most cases, it turns out, the songs are not remembered verbatim, word for word. Rather, broad outlines of the story are remembered, perhaps using visual imagery, and structural constraints of the song are memorized. This is a much more efficient use of memory than pure rote memorization of the words, using up far fewer mental resources. The importance of form in poetry, and in song, is that form is the critical feature that helps to recall lyrics.
The mutually reinforcing, multiple constraints that help us to remember song lyrics are principally rhyme, rhythm, accent structure, melody, and cliches, along with various poetic devices such as alliteration and metaphor.
The rhyming scheme we find in most songs constrains the words that can appear in the last position of rhyming lines. Even though there may exist several words rhyming with the correct word, semantic constraints will prevent most of those words from working in the context of the song.