Fact is that song must be written to be performed without music background, even best voice sound empty, hollow if singer chose wrong song. Other than that, like people say autotune is obligatory in modern music industry. This and ghost singer, anonymous people which sing instead of some star. One of my brothers are sound engineer, also know some other sound engineers. They tell me "interesting" stories about some celeb singers. But there are some really brilliant singers, sometime they sing just for fun and crew entrainment, you must hear this to believe.