TDS (Total Dissolved Solids) There is all kinds of things you are removing with partial water changes. People only go by Nitrates as an indicator because they are relatively easy to test for as compared to the others. In the same reasoning, plant keepers often test for Iron as an indication of what all their numerous micro trace levels are. Alot of the uneaten food and poop from fish contribute to algae growth. And one of the most common failings of new fish keepers is over feeding.
As a new aquarium owner, it is actually more important to do frequent water changes than for the more experienced ones.
In this way, not only do you "reset" the water parameters to fix any mistakes you are making, but also to have less algae growth, and, very importantly, to keep your tank water as closely similar to your tap water as possible.
Often the very best and easiest way to fix any mistakes(overfeeding, chemicals getting in the water, overdosing medicine, etc.) is to do a massive water change.
If you do small, infrequent water changes, the 'cure' will just as likely kill them as what you are trying to remove by doing the water change.