Interesting. I had heard that doing traditional publishing, you are still doing a lot of the marketing. The biggest benefit is that you aren't using your own money to do it. I'm self-published, and the coming up with marketing campaigns is the part that kills me.
That's a matter of some discussion. They're not going to run your social media for you, which is what people think of as marketing. They're probably not going to place ads for your books because those don't really work well enough for them to bother (you'd be lucky to break even with internet ads, especially if you cost out your time).
But they'll usually send your book to a roster of reviewers, although whether it's their A-list or C-list of reviewers will depend. And being in the catalogue means a much better chance of landing in bookstores, although books can languish on shelves too.
So the answer is yes, you'll have to do your own marketing, BUT if you do so that marketing should result in more sales and opportunities because of the publisher's connections in the industry. On the flip side, will those connections balance out the smaller royalties you get through a publisher? That's a difficult question to answer, and depends a lot on how well your book does. Weaker books that get through a publisher might do well because of an advance and getting their fees covered. And something like Harry Potter, could that have happened without a publisher?
But a strong mid-range book, with an author who knows how to approach reviewers, should do better going it solo.