Filtering the water to process ammonia/nitrite is just part of it. An amazing about of chemicals are a part of fish waste/respiration that need duluted in an enclosed ecosystem like an aquarium.
I was part of some experiments with mollies while in college and the filtering ability of plants for them in a small aquarium. In three identical tanks (20 gallon longs) with identical plant mass/lighting/feeding/stocking (8 male Sailfin molly fry per tank, about 1 month old so no chance to change the bioload), we did weekly maintanence on all three.
One got only 'topped off' with water to replace what had evaporated.
One got 25% weekly water changes.
The last got 50% weekly water changes.
All three has 0 ammonia, 0 nitrite and between 5-10 nitrates throughout the 16 weeks of the class. The plants took in enough of the available nitrogen so that no real change happened in the 16 weeks.
Those in the tanks with the 50% water changes had the most growth. The 25% water changes were about 25% smaller. Those in the zero water change tank had very slight overall growth.
The next semester, all 3 were maintained with 50% weekly water changes, to see if they would all max out at the same size.
In the end, the zero water change tank had fish that were 2/3 to 1/2 the size of the others.
The 25% caught up more or less, to the growth seen in the 50% tanks.
By then end, they were 9 months old, so adults.