These are full-size models of designs that are especially handy when a presentation of the final product is needed. You can create a mock up from scratch but this could take you a lot of time which you may not have. Preformatted mock ups will save you time and help you produce professional results within a short period of time.
To help us understand better, say you were making a certain application. A client you had is eager to see how the home screen would look on a device, say, a phone. All you would need to do to create a mock up would be to upload the design unto the mock up application and it will inform you on the best size the image would need to be in order to be a good fit. From there, any number of adjustments can be done to your satisfaction. Then you will be at liberty to download the image of your applications home screen, upon which you will probably be charged a small fee.
Another example of what your experience using a mock up would be like is when you are trying to design a t-shirt. You would be presented with a t-shirt whose features you can edit, say print, design or color. The best thing about this is that any change to any feature will only be one click away, allowing you to visualize as many possibilities as you desire in real time. This makes deciding on the final product and what resources to invest in much easier.
Below are some mock ups you can download from our website to make your designing process easier or for any other project: