Although acceptable global illumination results may be obtained by using the environment color as your basis for lighting a scene, using a solid color, or even a gradient map will still produce a only a uniform distribution of light.
The more I use HDRI the more I prefer it to other methods of GI. HDRI stands for High Dynamic Range Imaging. It uses 360 degree photos taken from a reflective sphere, a series of pictures with different exposure settings that capture the ratio between dark and bright regions as represented in the real world.
Mapped as a spherical environment, HDRI illuminates the scene with different levels and colors of light based on pixel information.
The result is more realistic than other methods of generating global illumination, and the amount of light can be altered through the exposure settings.