Last week someone asked me what a publisher does for you. Why go that route instead of self-publishing? My off-the-cuff answer didn't cover everything I wished I told him, so here's my second attempt. Don't you feel lucky? (Just to make things clear, I'll be talking about publishing with well-established Christian houses/imprints such as Thomas Nelson, B&H, Harvest House, etc.)
They take the risk from the author.
You don't have to put up any money. The publisher pays the cost of printing, etc. You actually get paid an advance, and if you don't sell enough books to earn it back, well, it's their loss. You don't have to pay anything back. Of course that's not the ideal, and it'll hurt your future chances of publication, but still. The risk is theirs.
They give you credibility.
They need to make money to stay in business, and, as it's their logo on the spine of the book, they need to preserve the quality of their brand. So there's a screening process. They choose projects carefully, then let it go through many stages of edits. They have an art team to design the packaging in a way they believe will most appeal to their target market.
They have a system for getting your book out there.
This is very important. You can't sell your book if no one knows about it, and you can only reach so many by yourself. They have representatives that talk to booksellers to get your book in stores and catalogs. They might place ads or pay for better displays in big bookstores. They might print up bookmarks for you to use, design book trailers, arrange interviews, post extra information on their websites.
There are some advantages when you self-publish through a reputable company, of course. If you have a speaking platform, this can be a good route to go. You have total control over the product. (But do you have the qualifications to be editor/designer/marketer?) You get to keep any profits. (But you have to pay all the costs, and most self-published books don't earn out.) You retain all rights to your story and you can reprint it whenever you want.
In most cases, however, you should opt for pursuing the traditional route. If that door closes, look at the reasons why. You may need to hone your craft, build a better story, or wait for a better time with that particular manuscript.