Clouds add a lot to a landscape photograph mostly by making the sky interesting -- Mountains in Yamanashi Prefecture JapanA bright blue, cloudless sky sounds like part of what would make for a very beautiful day, does it not? Well, if you are a landscape photographer nothing could be worse! I have heard it put that, "a cloudless sky is the enemy of the landscape photographer." If there are no clouds in the sky, then it is just an expanse of a mostly solid color with no visual interest, especially if it is a middle of the day plain blue sky. Even for sunset portraits, I definitely prefer to have some clouds in the background even though the sky is all orange, yellow and red.
So the next time you are thinking to make a landscape photograph, make sure there are some good clouds to add that extra element of interest to you image.