Some use cases are described in the blog post.

I have wanted a feature like this before and worked around it by replying to the email but sending the reply only to me.

an immediate one for me is to track notes from phone calls to support. My current process is to reply to the email with my notes from the call.

It's just another way of managing notes and tasks. Attaching notes and tasks directly to the mail, can result in shorter workflows. I mean, information should be where it's used, and mails are a very central point of some peoples' life.