Of course, photography is all about light. It's how the image is formed, but it is also what makes many images more compelling than others. You don't always need 'good' light to make strong images if the subject matter and/or formal elements suffice, but images that are predominantly about light do have a strong visual impact.
Close-ups of animals' eyes are a bit of a cliché, I've taken one of a horse in the field behind my house before. It's quite a nice image, but the one I took (of the same horse) yesterday evening is more striking. With the sun setting after a dull afternoon I wandered round the garden looking for a subject. The horse came to the fence, I saw the way the rosy sunlight caught it's head and did my best. If I'd nailed the focus on the eye on another shot it would have been a killer, but this one will have to do. ISO 6400, slight crop, noise reduction with Neat Image, light selective sharpening.
(Click the pic to see it better on a darker background)
The image is about light, but it's about shape too, and colour (which is light dependent), and focus, and horses, and... Like all photographs it's about whatever the viewer reads into it. But if it hadn't been for the light I might not have taken the shot.