Yes and this is what I was curious about. The nitrate levels should remain fine with 2 WC / week (<5) and ammonia and nitrite should remain 0 so I am wondering what makes this necessary - is it just a tried and true thing and its true for a variety of factors like those said above or is their a very specific reason that makes it necessary.
And as a separate question would 80% water changes twice a week work. Because a 80% water change twice a week would result in only 4% of the water being leftover every week (0.2 * 0.2) while a 35% day water change would result in 5% of the water leftover (0.65 ^ 7), so wouldn't the 80% twice a week water changes be noticeably better. (Though 70% twice a week would result in 9% of the water being left over)