They exist because clients have wild imaginations but zero ability to visualize how a flat design translates to real life. Show someone a logo on a white background and watch their face go blank.
Mockups bridge the gap between "here's your design" and "here's how it won't look terrible in the real world." They're basically training wheels for people who can't imagine a two-dimensional design existing in three-dimensional space, which is pretty much everyone who isn't a designer.