Off Page ranking factors are those that publishers do not directly control. Search engines use these because they learned early on that relying on publisher-controlled signals alone didn’t always yield the best results. For instance, some publishers may try to make themselves seem more relevant than they are in reality.
With billions of web pages to sort through, looking only at ‘On-The-Page’ clues isn’t enough. More signals are needed to return the best pages for any particular search.