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It's not just "darker", or with highlights.
Raindrops on a surface are gonna cast a shadow (as long as there's a hard light), but because water is refractive, and they're in a round-ish shape, the refraction is going to concentrate the light in certain areas and often create some sort of "glow" (from more concentrated light) in the middle, *on top* of also having highlights.
Water droplets can also create caustics, which means light hotspots can also show up in some of your droplets' shadows, under certain conditions. This is something the artist in the OP has actually depicted under certain droplets, which fucking amazes me considering how unremarkable every other aspect of the picture is.
