Mockups are almost a must-use if you are a designer (of any kind) when dealing with clients or even for you to preview how your work looks like in an integrated form.
Mockups are the closest form to your final design. Let’s say someone hired you for a design job. After completing the project, you show it to the client wrapped around a mockup. This way, he can see how the product integrates and also approves or disapproves it. This way, you will be sure that, after the client approves a final design, he will pay for your work (because a photo with the design on a mockup is basically useless to him as is).