It doesn’t seem like that big of a deal, but skyholes are important when it comes to explaining where the elements of your painting are sitting in space.
When we are composing elements in a picture, we are really making kind of an illusion. We are representing three dimensional objects on a flat picture plane.
I typically want to make the spatial relationships of the elements as clear to my viewer as possible. I want to tell my viewer what is in the foreground, middle ground and background. Essentially what is in front of what and what is behind what.
One thing that we see in a landscape is skyholes! If we get them wrong, then the spatial relationships get all mixed up. Our viewer doesn’t know what is in front and what is behind.
It’s an easy mistake to make, but it can quickly flatten our picture plane, destroying the illusion of space. There is more to say about skyholes, but that’s another lesson!