What are the advantages of mock$-$ups?

They are nearly a $1$:$1$ representation of the real system, GUI can be tested on different systems, and they are testable with real values.

They are nearly a $1$:$1$ representation of the real system, GUI can be tested on different systems, and they are easy and cheap to create and customize.

They are nearly a $1$:$1$ representation of the real system, very detailed tuning possible, and they are reusable for the productive implementation.

They fire the stakeholder's imagination of the future system and help to caln stakeholder's fear about project progress.