Just like every other job, some directors are better than others at certain aspects. Some are great with getting talent to perform, some are great with camera work, and some are visionaries. Just like any CEO/founder of a start-up, to have a successful film, you have to staff up with people that are strong where you are weak. A director that is great with talent but less in the camera dept hires a great DP. A strong Assitant Director can keep the day to day management going while the visionary director focuses on the 30,000' view.