Natural light is great if there is enough of it and it has a good distribution for your subject. But when it's quantity, quality, direction, or colour is not good enough, then supplemental lighting becomes necessary to fulfill your photographic vision. I've been photographing for over 40 years, and my biggest lack was understanding how lighting can and does shape the subject. Over the past five years, I've made a concentrated effort to learn (master is not yet within reach) how to use supplemental light, and how to read the light to see how much was needed and where. Properly done, you don't even know flash or supplemental light was used, unless you study the image very carefully to see the sources of lights and their shadows.
One source which I found to be very useful is:
Amazon.com: Light Science and Magic, Fourth Edition: An Introduction to Photographic Lighting (9780240812250): Fil Hunter, Paul Fuqua, Steven Biver: Books. Keith, one of the forum moderators, recommended it to me and I have read it at least three times cover to cover and am still working my way through the various exercises. Which much of it deals with studio work, understanding light, shadow, various types of reflections, and the way light interacts with your subject is equally applicable to other situations. I strongly recommend it.
Google "Off-Camera Flash" or OCF and look though the many references that you will find. Direct flash is usually a poor contributor to lighting a scene, and benefits from various strategies such as bouncing (against walls, ceilings), reflecting (umbrellas, softboxes), diffusing (shoot-through umbrellas, softboxes), etc.