Nowadays, the net is full of mock ups products pictures. According to the Wikipedia, “In manufacturing and design, a mockup, or mock-up, is a scale or full-size model of a design or device, used for teaching, demonstration, design evaluation, promotion, and other purposes”
It is a very good idea to create mock ups in order designers can show to possible customers how a design how would look on a kind of product.
Sales on line are increasing every year, as lots of online brands sell their products on it. Some of these POD or print in demand sites sell their products showing mock ups products.
I am licensing my artwork with some of these brands.They have a great professional graphic designers team who are the ones who make them.
Sometimes they are so well created that it is not easy to distinguish if they are real products or not.
Designers can improve their portfolios adding mock ups made by themselves.
There are lots of sites where you can buy mock up products templates, which it will make easier to create them.