Part of that expanded definition talks about the life processes of fish.
There are a million scenarios out there but the anecdotal one is always easier for people to grasp onto. If you can picture a storyline, then people understand this more clearly.
If you have a body of water that's a creek that doesn't have a single fish in it at all, but it flows into a creek, a river, or a lake that has a viable fishery in it, that upstream water, because it influences the life processes of fish, would also be protected.