The title of the post, which the submitter dutifully copied, is IMHO unfortunate since the post seeks to answer the following question:

What I need is to understand why it is designed this way, and to see concrete examples of use cases that motivate the design

It's not "just another" explanation for how OAuth does, which was my immediate guess when reading the title.

However glad I opted to give it a chance, and likely especially illuminating for the younger crowd who didn't get to experience the joys of the early web 2.0 days.