When doing water changes, two things are done simultaneously. The first and most important being the removal of nutrient build ups within the marine aquarium overtime. Build ups such as phosphates, nitrates, dissolved organic compounds and hormones that inhibit growth. Most hobbyists view their water quality based solely upon nitrates, however just because it is easy to test does not mean it gives a good overview of the marine aquariums quality. Your nitrates may be testing within an acceptable benchmark; however this can easily trick yourself into thinking that your water quality (saltwater/natural seawater/NSW) is far better than it actually is.
Secondly, another beneficial outcome of water changes is to introduce ‘good things’ that become depleted overtime. This can include healthy bacterias, vital minerals, trace elements, nutrients and life/coral food if you are using natural sea water (natural seawater/NSW). Additionally, the final outcome of biological activity is acidification to which depletes KH in your marine aquarium. As your KH becomes reduced the pH will then inevitably decrease, and this can most definitely result in tank crashes.
Lastly, another thing many hobbyists tend to forget is that water changes have a huge impact on your livestock capability. It would be inattentive to discuss stocking levels without considering the frequency of water changes you do and your natural sea water (natural seawater/NSW) quality. Many newbies into the hobby simply think that a standard rule of thumb calculator based on inches of fish per gallon and tank sizes alone is extremely flawed. Through various studies it has been proven that water quality is 4.5 times more important than aquarium volume when it comes to livestock health and growth. One study that really stands out is where two setups were examined, one being 20g that was heavily stocked but received large daily water changes, the other was a 50g tank with half as many fish and water changes once per month. The end result was that the 20g fish grew at a healthy rate and had significantly better colouration, whereas the 50g aquariums fish were paler and had a stunted growth rate and were more susceptible to illness. The findings of this study have also been supported by many aquarists who find that their fish are a lot more healthy and fat with greater colour the more water changes (natural sea water/natural seawater/NSW) they do (within reason).