Fiber is an interesting nutrient in that it's not really a nutrient. Fiber is primarily used in foods as a "filler" to give foods more substance and make you feel full. It also is important for regulating your digestive system, indirectly controlling weight, and lowering the risk of heart disease and cancer.
Because foods high in fiber take more time to eat, your body has time to signal the stop of the "hungry" feeling, and so you eat less. This helps you control your weight.
Some forms of fiber have been shown to reduce cholesterol levels in the bloodstream. One recent study showed that 3 ounces of fiber a day can lower cholesterol levels by as much as 20%! Another study showed that eating 7 ounces of carrots (high in fiber) every morning lowered cholesterol levels in the blood by 11%, and increased fat excretion by 50%.
Fiber is not without its downsides, however. Fiber can cause gas, and has been shown to deplete levels of calcium, zinc, iron, magnesium, phosphorus, copper, B12, and protein from your system.