This is an explanation I read a while back, I think it is reasonable.
Changing water does more than eliminate nitrate. It eliminates many
dissolved organic compounds (DOCs) in the water which over time
accumulate and harm fish. Nitrate levels are an indicator of DOC
accumulation. Nitrate by itself, and at modest levels, does not harm
fish and is essential for plant growth. A water change when nitrate
levels are high will dilute out the DOCs, however in some tanks
nitrates may need to be added back to keep the level high enough for
adequate plant growth. For tanks where low nitrates are not a
problem, many fertilizers do not contain nitrate.

Not the best rephrasing - read better from the original source... but
I can't remember what that is.