Hello, and welcome.
Like mentioned by almost everyone, you can use Medium as you see fit. So I can only really speak for how I’m using it.
I’ve chosen to put some of the things I write, things I think of as blog posts, in my own little blog-publication. There’s 3 main reasons for it. First, you can personalize a publication much more than your profile. You can change your colors, put up a header picture, and I really enjoy doing that sort of thing. It makes it feel more like my space. You can also feature more than one story, if you wanted. On your personal profile, you can feature only one. Second, the search options on Medium are still a bit clunky. If you want to find something you wrote a couple of months back, good luck finding it. By putting some of my stories in my own publication I know where those stories are and finding them is just a little easier. And third, the pieces I put into my blog-publication, I feel are just a bit too personal to put into a publication with multiple authors. (This distinction between a public-publication or a private-publication open to the public, is a personal oddity.) Medium has a rule that a story, once in a publication, can’t be published in any other publication. So putting my private stuff in my own publication is one way of making sure others won’t be able to request it, without having to figure out those settings for each story or giving off the vibe that I think it’s somehow too good for the requesting publication (which it certainly isn’t)
Just some reasons why someone would want to put a blog in a publication :)