It seems to be the case that many singers are great vocalists, but don't have a lot of musical training per se, and thus have some issues when it comes to the more technical side of basic musicianship, for instance timing.  Since timing isn't only important when you practice on your own, but especially crucial when you are part of an ensemble - choir or band - it's one of the skills you must work on continually in order to perform at your best. Because, let's face it: even if you have a perfect voice - if the timing is off and you just come in on the wrong count, this will be your most limiting factor of putting on a great performance.