"Inaction breeds doubt and fear. Action breeds confidence and courage. If you want to conquer fear, do not sit home and think about it. Go out and get busy."~ Dale Carnegie
This is the primary reason for writing practice. Whether you have a WIP (Work In Progress), or you are using random writing prompts, writing every day increases your confidence as well as your ability to take literary risks.
As I said yesterday, fling words at the page with abandon. Maybe only ten out of every hundred will stick, but those ten will be ten more gems than you would have had if you had spent your time just thinking about writing.
"Confidence comes not from always being right but from not fearing to be wrong."—Peter T. Mcintyre