All light is natural light. It's all made of the same thing - photons. The photons are just liberated in different ways by the different light sources.
The photographer has severely limited control over available light. Strobed light (flash) allows much more control, and controlling the light is the essence of photography.
Using strobed light adds a level of complexity to photography, but it's the way to make more professional looking photos in a broader range of shooting situations. Using strobed lighting allows exposure control of the ambient light separately from the exposure of strobed light (flash) with a single shutter release.
Ironically, using the camera's automatic, flash TTL modes is much more difficult to learn, and get consistent results from, than using both the speedlight and camera in manual modes.
Another difficulty many have is they lack an understanding of the basic technical fundamentals that make photography possible. In other words, they don't know how their tools work, and are consequently not able to use those tools effectively.
The more automated dSLR photography hardware becomes, the more technically astute photographers must be to understand and allow for the limitations and nuances those automatic features have.
A case on point is understanding the limitations of auto focus. If there is little or no contrast in a scene, the passive type of auto focus most dSLR cameras use today just cannot function properly.
