The writer's purpose is to speak the truth. Our goals is to write so well, people hardly realize they're reading. The words just flow together like a train of thought, like a conversation. The writing literally springs from the ink on the page to the thoughts in your head. It should be done so well that you have to stop and realize you're actually reading and not thinking. This is the essence of beautiful writing. This is writing that impacts people, holds power, is memorable, and might even get published (not that publishing really matters).
Sometimes writing can be frustrating. It's difficult to transmit thought into written word and back into thought again with smooth, flowing syntax. Like a poor singer, sometimes writer's words are off key, flat or sharp, and the reader doesn't know why the work sounds bad, they just know for some reason it does. This is where the shit detector has to come in. As writer's we need to be humble enough to realize when our words don't sound good, and determined enough to rearrange them until they do.