Chemical Oxygen Demand is an approach for measuring the organics in water. To be organic, the substance has carbon as a component. When an organic is oxidized, the carbon eventually becomes CO2. So, there is a test protocol that allows a measurement of the level of organics in water by seeing how much of an oxidizer is required to convert the carbon to CO2.
The point is that high COD means there is a high level of organics. As we all have learned, getting organics out of the water is a good thing. Water changes and foam fractionating are two techniques. Dumping waste from settlement chambers can be seen as either getting the organics out, or getting rid of a source of organics. Aeration and otherwise maintaining maximum dissolved oxygen levels contribute positively by hastening the natural oxidation.
Some folks like to test and monitor everything. For them, a COD test kit would be a wonderful present to give on some special occasion. They can have many happy hours of playing 'mad chemist in the garden'. For those who never get around to doing the testing they meant to do some day, the key thing to my mind is that getting rid of organics in the water column is important. The more you can do, the better your fish will develop. This is what maintaining the water is all about.
...And, yes, in theory one could go overboard in eliminating organics; but I don't see that as worth worrying over unless the person is one of those who relies on dosing potassium permanganate every week.