As a couple of other posters have said, there is a point to the cliched opener. It gives the listener some idea of the kind of song it's going to be, and also gives them time to settle in and get used to the sound without missing some vital part of the story. I think it's probably most useful when a song is unaccompanied - if there's instrumentation, a few introductory bars can do the same job.

Then the singer can introduce the important (or interesting, or surprising) elements once s/he has got the audience's full attention.

Marje