The mockup is a mid to high fidelity static picture. The aim of it is to demonstrate the color schemes, content layouts, fonts, icons, navigation visuals, images, and the overall feeling of the future software product design and user experience.
Mockups are realistic. It is a perfect opportunity for you to see how all your design decisions play together. What if the color scheme does not work with your shapes? Mockup is the way you can have a look at the final product even before the development starts.
Mockups are easily revised. It is easier to make changes in mockup tools at the mockup stage rather than later in coding. Your developers will appreciate the fact that they do not have to revise the product design.
Mockups are convincing. If you still need to win the trust and affection of your project stakeholders, showing them mockups is what you should do. They are intuitively understandable and closely resemble the final software product.