What is it?  How does it work? 
The important question to ask is,  What can it do for me?

Put simply, CGI can help you to communicate an idea, design, product or process to a wider audience.
The viewer does not necessarily need to understand how the image is created, merely to appreciate the subject in question.

CGI can provide you with an image where photography cannot.
When the subject is difficult or impossible to access with conventional photographic equipment, CGI can provide the solution.

If the subject in question does not exist, except for design drawings, sketches or even just ideas, CGI provides a visual communication tool.

When conventional photography is too restrictive for the creative process or there is a need to have more control over the subject matter, CGI is the answer.