I would say that one of their main advantages is that they allow asynchronous communication. That means that the information provided can last through time and space, the reader or viewer doesn't need to present at the time the material is developed. That is the reason we can enjoy books that were written hundreds of years ago or that someone in China can watch the same video as me.
As a result they both are great for communicating a message, information or telling a story to entertain, they reach places where other media is unaffordable.