What exactly is Content Negotiation? According to Wikipedia it’s “a mechanism defined in the HTTP specification that makes it possible to serve different versions of a document at the same URI, so that user agents can specify which version fit their capabilities the best”.
photo by JD Hancock
So, it serves two purposes: (1) making it possible to have different versions of the same response, and (2) letting clients specify which version they want to receive. Usually, this technique is applied when there are several types of user agents consuming the same HTTP resource but, because they have different rendering capabilities, they might ask for different content types.