The exact purpose of water changes is to replace polluted water (fish wastes, decaying plant matter, leftover food) with clean water. This is probably the most critical aspect of tank maintenance, in my opinion--and that is based upon those who have kept fish for years.
Doing often and regular changes reduces stress on fish, disease, ailments, and makes them darn awfully happy!!! Even with frequent water changes my fish swim about as if I'd just dosed them with happy water. Generally, the more frequent, the less water changed out, but I"m sure there will be other opinions.