Dictation focuses on bottom-up processing. As the sounds enter the ear, the listener uses his knowledge of grammar and vocabulary to decode the message. This linking of sounds to internal knowledge can lead to an improvement in listening ability. There are several reasons why songs can be useful. First, one of the main reasons that teachers do not use more dictation is that it is so unpopular with students.
Students get bored very quickly.
I remember in my own high school days letting out a groan with the rest of the class every time our French teacher decided to do a dictation. Songs are a way of disguising dictation behind something that students enjoy listening to. The second reason is that the melody and rhythm of songs help to maintain the natural rhythm and stress of speech which can be easily lost by dictating sentences at a slow pace. Finally, dictation is a great way of presenting a song and a good start for all the other activities which use songs such as discussion, vocabulary work, or just singing. There are several ways that song dictations can be carried out.