Water changes are only *really* necessary when you're EI dosing, and you need some way to make sure plants get all the ferts they need while keeping them all from continually increasing, in which case you do 50% weekly.
Other than that, however, water changes will reduce the stuff that isn't getting regularly consumed, thus cleaning up the water column, which benefits fish. And, to a lesser extent, plants.
It also clears up the water a bit, which benefits you, if only aesthetically.
One other thing - there's no reason to try to keep nitrates low. It's not a reason to do or to not do water changes, but you mentioned keeping them low, which suggests you might have the wrong idea about them. Low isn't bad, unless they're close enough to zero to possibly bottom out, in which case, your plants will be upset with you. Better to keep them high enough that there's no chance they hit zero, even if you do water changes.