CreateSpace has free ISBNs to offer authors. The ISBN is the unique identifier used in all traditional systems for identifying books. Amazon has a unique number of its own called an ASIN. The person or company that holds the ISBN has the publishing rights to your book, at least in that edition. This was usually the publishing house. In effect, when you buy your own ISBN (Bowker.com), you are now the publisher of your book.
ISBNs cannot be transferred or resold. So any vanity publisher like LULU or CS is not giving you an ISBN and they actually hold the publishing rights to your work, in that edition that is identified by the ISBN.
I originally started in 2007 with Lightning Source. I bought a package of 10 ISBNs and published a few books under my own ISBN numbers. However, when CreateSpace came along, I realized that my publishing process needed a way to submit changes without incurring additional charges. And then, a couple years ago, I decided to redesign and retitle all my books. Some of them I had to retitle several times, since I would get them out on Amazon after a week or more of work, and I already had a competitor with almost the same title.
ISBNs are linked with the title, subtitle, the physical size of the book, and the number of pages. Every time you make a change to one or more of these elements, you have to publish under a new ISBN. So, these changes would have cost me not only time, but money and my precious ISBNs!
This is why I publish under CreateSpace now.
The advantage in using your own ISBN numbers is that you “own” the publishing rights in that edition. However, if you make a lot of changes to your book after you have approved it for publication, you might want to use free numbers.
FreshAsylum 
I’m still trying to wrap my head around this ISBN thing. If you buy your own ISBN, or group of ISBNs, are you legally permitted to publish your book in any form with any publisher you choose? If Amazon gives you an ISBN, does that mean they own all rights to that version? Is their “ASBN” interchangeable with an ISBN?
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ISBNs have nothing “legal” about them. They are just a traditional way to identify a book. However, whoever purchases them “owns” the publishing rights to that edition because they are the official owner of the numbers. You always own the copyright. ASIN is Amazon’s internal Kindle book identifier. Don’t worry about ISBNs or ASINs; the publishing industry is in flux, and changing daily. You don’t need an ISBN for a Kindle book. The important thing is to get your book out there!
More about ISBNs: http://www.isbn.org/
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