Changing 25% of your tank's water every other week serves two purposes: it dilutes and removes nitrate before it accumulates to dangerous levels, and it replaces trace elements and buffers that get used up by bacteria, plants, etc. Finally, regular partial water changes help insure that your tank's water chemistry doesn't deviate significantly from that of your tap water. The latter benefit is especially important should disease strike your tank; water changes are the most important step in controlling disease, and large water changes are not safe unless the chemical composition (e.g., pH and GH) of your tank's water is similar to your tap water.