Mockups are an essential part of the creative process. They are not just a great way to get client feedback but also to help clients visualize how the design would look in real life. Living in a world that is changing quickly, it is essential to iterate ideas flexibly.
Mockups allow the designers to test their design and make changes to make sure that the design is fit for production. Additionally, if you are doing concept projects which do not involve developing products, displaying your designs on mockups gives your project a sense of professionalism.
Though, the greater the number of mockup sites on the web, the longer it takes to find one which has high-quality mockups. As a graphic designer, I prefer to work with a handful of websites that I trust to provide good mockups for free. These websites have a great variety of mockups for all my branding-related needs, from packaging to stationery.