In painting or any kind of discipline, you're either moving forward or slipping backwards, there's no standing still. Staying in a comfort zone in your painting or subject matter is a good way to become stagnet. Pushing yourself beyond what you think you can do is important. Even if your attempts fail most of the time, you're still moving ahead. It forces you to try different techniques, solve problems and come up with solutions you hadn't thought of before. It also forces you to study different paintings, to see how other artists solve the same problems.