In my mind, two of the major benefits to traditional publishing are promotion and distribution. Even if the author still does a lot of their own promotion, the publisher will likely do some promotion through their catalogs and such. As someone mentioned, the publishing house's label can add some credibility to your book. Also, a major publisher has the means and connections to distribute your book to stores across the country or even across the world, as well as online. A self-publisher, especially one working with print editions, may not have these connections and will have to work a lot harder to get their book in stores.
Another benefit to traditional publishing is the fact that the publisher takes on some of the work of getting the book ready for print, leaving the author to focus (more) on writing. Formatting, cover design, etc. are things the publisher might handle, and the author doesn't need to have the know-how or time to deal with them.
On the flip side, self-publishing offers ultimate control. The author has direct control over every aspect of the book, which is something we often do not have with traditional publishing. With traditional publishing, the author may not have a choice in cover design, illustrator, and so forth. Self-published authors do, and they also retain all the rights to their work.
I have a friend that loves to do concept art for me. By self-publishing, I can use these illustrations in my books. With traditional publishing, that probably wouldn't be an option.
Similarly, self-publishing can be friendly to books of unusual format or length, or books that are from an obscure genre or cross-genre. If an author has trouble finding a publisher who markets books of her obscure type, she may have more success self-publishing and promoting directly to her niche market.
An obvious downside to traditional publishing is the hurdles you have to cross to break into the market. Convincing someone to buy your work can be incredibly difficult, and it may take a long time. There are no such hurdles with self-publishing, and you can do it on your own time frame.
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). You'll likely spend quite a bit more on marketing, which you might end up doing with traditional publishing anyway, but even that can be done at a fairly low cost. It takes some looking around, but I've found a lot of printing websites that are low cost. I've probably spent under $300 in marketing since I started, but still have some great marketing tools.
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A big downside, which is also an upside, to traditional publishing is agents. Agents are hard to get, but when you have one they can really do the publicity angle of getting your book published