Reddit reviews The Hidden Origins of Islam: New Research into Its Early History

>18- If the Quran is perfect and clear, why is the hadith necessary to explain it and supplement it?

I'm reading a book called The Hidden Origins of Islam and one of the explanations given in it for this phenomenon is that the Quran might actually be a compilation of older texts from various religions and sects-- that's why it's so disjointed when you read it. It constantly changes topics as if a person with ADHD wrote it and contains major contradictions-- not to mention all the scientific errors. The book proposes that in order to make sense of this chaos and give it a context, the ulema and/or political authorities of the 7th-10th centuries had to invent the concept of hadiths to expound on Islam and give the Quran better continuity. This would explain why most, if not all, of the hadith collections were written 100-200+ years after Muhammad's supposed death.

Edit: rephrased second-to-last sentence

This text has some decent info form what I've heard.