Without light, nothing can be seen.
Thus it is extremely important. And beyond the basic requirement for sight, lighting is equally, if not more important for appeal.
To be a competent and/or skilled 3D artist, you need to understand first how you can introduce illumination into your scene. This can be accomplished with lamp objects like Spot lamps, Sun lamps, Area lamps, etc. You can also assign an emissive material to non-lamp objects so they illuminate as a light source.
Lastly, you can illuminate your scene with the World environment. Each of these are explained in the "Types of Illumination" chapter. After you know the basics of how to illuminate, the next step for a lighting artist is developing appealing illumination. The baseline light setup is called "3 Point Lighting".
The "Application" chapter covers this as well as how to illuminate your scene entirely with a single HDRi environment texture.
CC Music: "Outer Planet" by Robbero