Remember the fringe of backlit chestnut leaves along the bottom of the big tree? Of course you do. I think everyone of us saw what happens if you don't give adequate emphasis to the parts of a scene that describe the light.
Paint is not light. It is bounded on both ends of the value scale by inherent limitations. We cannot make any part of a watercolor brighter than the white paper, and we usually don't want to apply darks so thickly that they remain shiny after they dry.
There are often specific places, like the fringe of glowing leaves, where we must take care to reserve the lightest lights. Sometimes we also need to exaggerate the difference between those extreme lights and darks and what is immediately adjacent to them.
The horse's crest is brilliantly lit partly because it is surrounded by strong darks. If you failed to reserve the sliver of pure light the painting simply would not contain the same quality of illumination.
I know you know where the essential bits of light are in this scene. Once they're covered up, they're gone for good, so we have no choice but to be careful right from the start to reserve them. Draw them, mask them, whatever it takes. Just don't let them get away!