Light is required in order to stimulate the photosynthesis process that is required by most plants to produce a fruit or crop (rhubarb is a notable exception to this rule). The spectrum of light the plant requires changes over its life and this needs to be managed as part of the systems you put in place.
The amount of light changes thought out the life cycle as well (depending on what you are growing), if the plant comes from a region away from the equator, it may require 18hrs of light in order to grow and prefer white light for this period of growth. During flower or bloom periods (and often in order to stimulate the change from growth to bloom) the plant may only require 12hrs of light. This cycle reflects the changing amount of light made available to the plant in nature through the natural change in day length.
If you are growing using the Sun you have everything you need as far as spectrum is concerned. However, you may not be able to get enough power of light consistently throughout the life cycle (lumens), or conversely, you may get too much (winter v's summer).
One of the contradictions that the grower must resolve is the need to get the lamp as close to the plant as possible in order to maximize the amount of Lumens (strength of light) made available, but if the lamp gets too close the heat it generates can scorch the plant and this, in turn, creates significant problems including shock and wilt.
