For one thing, you can't manage files over the same protocol you use to serve them, with HTTP, without extra application-layer add-ons (WebDAV or custom web file management junk) FTP can both serve and manage, all over one protocol.

"But why wouldn't you use some other method to manage your files? Why combine the two?" I dunno, but WordPress is basically that (managing your blog's/site's appearance, content, and server-side plugins, over the same interface/protocol that serves the blog/site) but for blogs & websites, and it's damn near the most successful Web project ever, so there must be something to it unless that's not a big reason for its success (and I'm pretty sure it is).

I can certainly see the appeal if your main focus is serving files, or providing file-serving hosting to others (say, other departments, or to paying clients, or whatever). One daemon to configure for the whole task.