Lighting isn't just exposure, this is one of the annoyances of highly sensitive cameras these days: you have amateur shooters making incompitent statements like: "My camera can shoot iso 4000 and i my lens is f0.95, i don't need lights"
Lighting is not exposure, lighting is design. In some cases, existing light can be used, but no matter what the case, if you're only looking at getting the frame or part of the frame to correct exposure level, then you're doing it terribly wrong. Dramas should be lit differently than comedies, which should be lit differently than noir, etc... Learning about how to achieve different styles of light is critical to getting your film to look professional.
This video from Full Sail University does a great job of presenting one of the basics of lighting setups: the 3 point setup. And they do it in a remarkably short amount of time.