Songs stick in the head. I once taught a song from this book in a class, as a substitute teacher in a school. I was next in that school a year later, and one of the kids saw me in the corridor and started singing it. Being memorable is such an important quality of any grammar teaching material. Add to that the pronunciation benefits, and it makes songs ideal for language teaching.
I wrote Singing Grammar as my second book, after Pronunciation Games. I got the idea after reading in David Crystal's Encylopedia of Language (CUP) about certain tunes which are common across many different cultures, such as the taunting melody known as the 'catcall'. I used this as the tune for the first song in the book, and then went on from there.