These organic substances can have various deleterious effects on the aquarium inhabitants ranging from reduced growth, to reduced disease resistance, to metabolic stress. In some cases these organics are mineralized, by bacteria present in the tank, into ammonia. The ammonia is then oxidized by the nitrifying bacteria we are all familiar with to the final product, nitrate, which then tends to accumulate in the aquarium. Unfortunately, many of these organic substances are not mineralized and tend to build-up in the aquarium. That is why water changes are usually advocated.
Many people think that water changes are designed to lower the nitrate concentration.
While this may occur to some extent, the real reason is to lower the DOC content of the water. Since nitrate and DOC concentrations are often directly related, and nitrate is easy to measure, it is used as a yard stick to determine when to make a water change. The thinking then goes that if we could remove much of the DOC before it accumulates or becomes converted into ammonia, we could reduce the need for water changes, lower the stress on our filter, lower nitrate levels and improve the growth and health of our organisms. These then are the primary reasons behind the use of chemical filtration. At this point I would just like to clarify that I am not saying that if you use chemical filtration, water changes will no longer be necessary.