I found this interesting explanation on Ernies Page at http://www.lingolex.com/userpages/music.html
He also has a great list of songs you might want to use with older students.
Songs, according to many scholars, are among the best ways of teaching a foreign language. The authors of Spectrum (Prentice-Hall Regents Publications) state that "Songs are an important aspect of culture, representing the history, folklore, and current idiom of a country.(...) Singing can build students’ confidence by allowing them to enjoy a degree of fluency in English before they have achieved it in speaking." Also,songs can be incorporated to all language skills ( listening, reading, writing and speaking).
This is possibly the way that most teachers use songs. Choose a song that has some connection with the structure or part of speech that you are teaching. Delete a few words from the lyric and hand the incomplete lyric to the students. For lower levels, you can include the deleted words in the bottom of the page ( of course, out of order) Hand out the incomplete lyric to the students and play the song a few times, depending on the level of the song. Students listen and complete the missing words. Then hand out the complete lyric (or write the missing words on the board). Give the students some time for correction and answer any vocabulary questions. Then play the song again , asking the students to join in and sing (they might not be aware of that, but by doing so they are actually practicing pronunciation and stress)!!
As a follow-up, you can prepare a sheet pointing out a grammatical point that you might want the students to learn at that point, and have a structural or communicative activity after that. Some great songs that can be used for specific grammatical purposes: