This principle is the prominent one in UI design theory. If a UI is clear, you can easily understand what it was designed for, what you can do with it, and how to do it. Have you ever been blocked from looking at a web page or piece of software for several minutes, without understanding how it was supposed to work? Maybe it was cozy, full of controls, texts, and colors which did not suggest any path or flow to follow.

What did you to with that web page or software? I can be pretty sure you abandoned it, concluding it could not be a useful resource if you could not even understand how to use it.

This short exemplification clearly explains to you how relevant clarity is in design: we as humans tend to be discouraged by things we do not understand how to use.