Publishers can help with a lot of book-related things--typesetting, printing, cover design, and so on--that authors don't necessarily know how to do themselves. Automated print-on-demand services get you most of the way there, but professionals can still do a better job a lot of the time.
The real problem is the money to pay for it all. Most authors can’t afford to gamble four or five figures editing, proofreading, promoting, and distributing a book that may show nearly zero return. Think of publishers as VC for books.
Publishers don't necessarily do a lot of structural editing. Copyediting (proofreading), yes. But that's more like three figures to pay someone to do (which you really do need to do). Non-fiction editors will often also pay for someone to do a technical edit.
Promotion. For the average book, not much. The main promotion/benefit for, say, technical non-fiction is that you're in their catalog and will be on display in venues like trade shows.
Generally speaking, publishers aren't paying for book signing tours, publicists, etc.
Distribution these days isn't a big deal. Everyone can be on Amazon.
