John Constable, landscape painter of the 19th century in his letter to his friend John Fisher: “I am most anxious to get into my London painting room…I have done a good deal of skying, for I am determined to conquer all difficulties…The landscape painter who does not make his skies a very material part of his composition, neglects to avail himself of one of his greatest aids.” “(Skies) must and always shall with me make an effectual part of the composition…The sky is the force of light in nature and governs every thing.”
Constable was talking about oil painting here. The sky is an important element in landscape photography as well. The photographer must pay attention to what is going on with the sky in order to make successful and attractive landscape photos.
A bald sky, even if deep blue, makes for a boring photo in most cases. I have made photos where the complementary color of, say, Winter broomsage offsets a blue sky nicely. This is the exception, however. Clouds can make a photo much more interesting.
Constable’s quote above about the sky being a force of light in nature is well taken - even in photography other than landscapes. They sky is the largest outdoor light source and it obviously affects objects on the ground. An overcast sky makes a great light source for portraits. A beautiful sunrise or sunset can make a great light source for portraits or other photographs on the ground. Often when other photographers are pointing their cameras at the sunset, I have my back turned to the sunset and am capturing subjects on the ground bathed in the beautiful light.