Confidence is an important thing for an author, but then, it is also an important thing for anyone in any line of work. When you're confident, it expresses itself in the work, and the reader can feel it, though I would argue that a lot of readers don't even notice it. Perhaps, also, the authors. But, if you've read a lot of new authors, taught in enough classes with men and women who are struggling to find their voices, you'll know the difference, the confidence that is required for an author to put down words, construct a scene, establish a theme, and draw the reader along, for whatever purpose that exists.
A lot of the time, I suspect that the earlier years of learning the craft based aspect of writing is, in part, about establishing the foundation of that confidence. There is, I know, a debate on if you can teach writing, or you cannot, and I've always fallen firmly in the belief that you can. As an author, you have to learn how to construct scenes with imagery, learn how to have dialogue sound real, learn the various ins and outs of the genre or form that you're reading. You can teach all that. What you can't teach is original thought. though you can support it. But originality and, to a degree, voice, come from an internal part of the author, and it is your responsibility as an artist to feed that, to allow for your creativity to have a range and depth, and since there are a huge variety of types of creativity and creation, there will be some people whose interests and mind lean towards fiction, and those who don't. Though people may put a judgment value on that creation aspect of the mind, there really isn't one. Write a novel, propose a theory in science, compose a song, build a car--not one of these endeavors is worth more or less than another, unless, of course, it is math.
Easy jokes at the expense of math aside, there's a certain part of the author, and not the teacher, I believe, that has to nurture confidence as he/she continues to write and publish. How the author does that, however, is a different question--and one, I suspect, that individual's approach differently.