That is a bit like saying "I never understood why people who can self-host video put it on YouTube". Most times your content will never reach a significant audience (if this is your goal), platforms like Medium/YouTube/Flickr/500px make discovery easier.
You're going from a simple CMS to video hosting is like saying why ride a bicycle when you can build a lamborghini from scratch. Sure some people have the bank roll to try and launch a youtube, very few, but everyone can afford to spin up a wordpress or some other sort of CMS on a one click host.
Slightly better discoverability and easier “subscriptions” are the main values of Medium for me. I find syndication ok at attracting readers to specific posts but not great at getting people to keep coming back (even with rss available).
