It used to be a dramatically better API than what the system provided. These days it's debatable as to whether it's actually better than just using NSURLSession directly, but AFNetworking is well established as the default that everyone who has ever touched an iOS application knows how to use, and there isn't really any incentive for ripping it out of existing applications (and if you have a mix of old and new applications it's easier to just use the same networking API in all of them, so it continues to be used for new ones as well).

The most evident pro is saving you to type many more lines of code to implement the download or upload of a resource (json, image, etc.) from/to a web server.