Just thought I'd throw this out for something to think about.
An aspect that some developing songwriters may not have thought much about before...

A number of comments in my most recent song in the Showcase (Where Do You Start), have been about the "beat" of the vocal - as Tangmo referred to it - which I think is a good way to think of it. Others mention the vocal "phrasing"...

I often refer to it as Vocal Cadence or Vocal Rhythm. The vocal - or the voice, itself, actually - becomes part of "what makes a song work"... a main instrument in the sound of the song....

...which is an important aspect of Songwriting. Much of Pop music is more about this aspect of a song than any other part. Rock, too (Folk is seldom about this. Singer-songwriter, depends on the singer/songwriter).

Typically those who are best at it are, in fact, also singers and/or musician. It is a rare (talented) "lyricist only" who can get that across (in a lyric alone).

There are many good examples in Pop radio. Here is one of the all-time best... Pay attention how the "singing of the words" falls on your ear. When done right, little else is needed...