Depending upon your view or experience, some singers really add a level of emotion to a song and are the means of connecting the words of the songwriter to the masses. However, there are many songs whose words do the connecting to the masses more so than the delivery. I find it difficult to make a generalization as to which is more important because music is subjective to each listener.
If you are referring to money, some singers may get more money simply because they are more aggressive or astute in asking for the compensation they want. Also, singers are the "face" of the songs and many work via several means practically every waking hour to promote the song...often more so than the songwriter. Many songwriters have the full potential to become multimillionaires with just one or two songs that top the charts but it is their skills at negotiating their take that matters most.