But why would you use a separate post format? I don't get it. It's all posts.

Most likely with a video or audio file you would have an intro bit of text and other elements on the post anyway other than just the separate post item. You can embed all those things INTO a post.

I still don't get why you need a separate post type. Please explain. Why would this be useful?

I can see a lot of users being confused by this. They will click on these new post UI buttons thinking they are insertion buttons for media items so they can insert an image, video, audio file etc.

I'm sure there are technical and developmental advantages to this but from the average end users perspective this is confusing.