people simply use the answer to keep nitrates down.... that's not the only reason for water changes. there are dozens of organic compounds that are the end results of waste breaking down in our aquariums, nitrates are just one of the easiest to test, so is used by most fish keepers as a way to test for buildup. not nearly as accurate in a heavily planted tank since the plants consume the nitrates, but a lot of the other organic simply sit there.
discus aren't the only fish that use hormones to slow the growth of competition. several other species do as well, just maybe not to as spectacular a level as discus.
larger water changes can help maintain a balance in your water parameters keeping them stable. where smaller water changes just slow the change. depending on influx versus export you wind up with a lowly stacking scale of parameters building untill you get "the crash" (be it appearance of algae, unexplained fish deaths etc). there is no simple explanation why some tanks can seem to go indefinitely on little to no water changes and other seem to require more. Often IMO its merely the cumulation of un-testables when you have several factors, that alone wouldn't be a problem, with less than ample water changes you get a low build up to failure levels.
and there is also inorganic build ups from heavy metals, and other things in water supplies. if your using the same water to top off that your using for water changes you are accumulating even more due to top off only adding and evaporation removing nothing but water so everything that is in your top off water stays. you would be amazed at the trace amounts of such things as petroleums, medications, and other nasties found in municipal water supplies. whikle safe for human consumption at the levels they are found can slowly accumulate over time in a closed environment like our tanks building up to fish fatal levels.
I created a very simple water change table a couple years ago that showed the results of various amounts of water changes on a given parameter, it was amazing that some water changes people were considering sufficient werent lowering or maintaining the overall content of the water they were simply slowing the build up rate, and this is where it grets people in trouble, they see "my nitrates are fine, but my fish are dying, but they have been fine for 7 months" and its simply because it took 7 months for the factors to build to toxic levels.
Unfortunatly when I crashed out my computer I lost all my articles and tables I had created on aquarium husbandry, most of it marine tank related but a lot of the information goes both ways.
just my 2 cents
"[Wabi-sabi] nurtures all that is authentic by acknowledging three simple realities: nothing lasts, nothing is finished, and nothing is perfect." -original author unknown-
125g marine reef tank, 6 gallon fluval edge minimalist aquascape (in progress), 1 gallon aqueon evo 2 betta tank.