Mockups can help the designer to uncover visual elements that clash before you arrive at the final, end design. Visual hierarchies, that show the difference between design elements such as font and color, should be determined at the mockup phase of the design process.
With the visual elements more refined than with wireframes, mockups allow stakeholders to actually review the visual side of the project.
A PSD mockup is a visual way of representing a product. While a wireframe mostly represents a product’s structure, a mockup shows how the product is going to look like. But still, a mockup is not clickable (just like the wireframe). As opposed to a wireframe, a mockup is either a mid or high-fidelity display of design.
John Deo Mockup | PSD Mockup 