Good magazines do publisher fewer new writers than do book publishers. The short story markets, at least for paying magazines, is much, much tougher than the book market. It isn't a matter of credits, it's a matter of competition and number of slots available. Because of the competition, a short story must be darned near perfect to sell to any good magazine. A book is long enough to have a flaw or three, and still get accepted.
Credits can help in both areas, but they actually help more in the book market than in the short story market. They help for four reasons. 1. Credits show you can write well enough, tell a story well enough, to make someone give you money for your writing. 2. Credits get you past lower tier reader, and into the hands of someone who can actually buy your story. 1. Good credits mean there's a chance you already have something of a fan base who will follow you to the novel market. Sell two or three short stories to high circulation markets in the same genre as your novel, and it doesn't take very many of those magazine readers to make the difference between profit and loss with your novel. 4. This relates to a couple of the others, but it's important enough to list independently. Track record. No matter the market, a track record of success is more impressive than a track record of failure. And agent or an acquisition editor has an excellent selling point when pushing your novel if she can say, "He was a success there, so it stands to reason he can be a success here."
This does not mean you have to have credits to sell in the short story market, or in the novel market. Good writing, good story, and good characters always win out. The fact is that most first novelists do not have good credits, simply because good credits are extremely hard to get. This is what makes them good credits. If anyone could get them, they wouldn't be worth having.
It just means that if you do have good credits, you're a couple of steps ahead of the pack.
I've made at least two novel deals over the years based on the fact that I had very good short story credits, and potential built in fan base was the deciding factor with the editors.