We don't learn much about the physical surroundings in this story, but—considering that rotting crabs are involved—we get the sense that it's not too beautiful
That's why the descriptions of light and dirt are so surprisingly beautiful. And because they stand out so much, we know they're important. You may find it strange to find light and dust together, since they're usually not related, but check out the way they show up in these quotes:
Sea and sky were a single ash-gray thing and the sands of the beach, which on March nights glimmered like powdered light, had become a stew of mud and rotten shellfish. (1)
"Glimmered like powdered light": beautiful. More like this, please!
"Stew of mud and rotten shellfish": uh, we'll pass.
Or check out this description:
He awoke with a start, ranting in his hermetic language and with tears in his eyes, and he flapped his wings a couple of times, which brought on a whirlwind of chicken dung and lunar dust and a gale of panic that did not seem to be of this world. (8)
"Chicken dung": no, thanks.
"Lunar dust": and, back to the beautiful.
So what's the point of these contrasts? We think Gárcia Márquez wants us to take the time to see the beauty in the ugly and ordinary. That's what Pelayo and Elisenda can't do. When they look at the old man, all they see is a dirty, homeless creep—even if they call him an angel.
But our narrator can see that he's beautiful as well. He may be crazy and off-putting, but he's a living creature. And, thanks to our narrator, we can see that, too.