It is easiest to sing when you have mastered the art of listening. Listening and mimicing sounds go hand-in-hand. In fact, that’s how we learned to talk…we copied what we heard. It is easiest if the singer has developed this art at a young age.
That’s why I always encourage my piano students to sing the rhythm or words as they learn a song. The goal, of course, is to match pitch. Later on, to match articulation and dynamics.
I don’t think we can under-estimate the value of learning a musical instrument and its’ correlation to learning to sing.