Water changes are used to replenish the elements that have been used up by your livestock, algae's, coral etc... the other use's for water changes are in the events of high nitrates or emergency's.
The chemistry section is got alot of articles going into more details about proper levels, what to test for, how much to dose, etc..., and some of it can get confusing. That's why I mentioned a calcium reactor earlier since to me, this looks to be the simplest/laziest way to maintain our water chemistry not to mention that you'd be adding stability to your water which our coral/fish appreciate also. You could literally go months without a water change and alot of people do.