Books in Print lists about 28 million ISBNs that are actively being bought and sold in the US, UK and Australia.
This, of course, does not include books with no ISBNs, and is not a global figure. But it's enough of a number to emphasize the point that there are an awful lot of books out there. And a self-published author, without the marketing and PR engine of a large publishing house behind her, or the funds to take out TV advertising time, desperately needs her readers to find her book amid these millions of others.
more and more shopping is done via search. If your metadata reflects the keywords that are being searched on, your book has a better chance of being found on the search engines (which link to your bookstore listing). The search engine is now the primary intermediary between your book and its reader.
The Book Industry Study Group has a list of 31 "core elements" of metadata - this is a good guide to what both online bookstores and search engines look for in a book listing. If your book is not sufficiently described in a way search engines understand, you run the risk of the book not coming up as readers are searching on the topics you're an expert in.
It's also important to remember that there are many sources of book metadata. In addition to Bowker's Books in Print, book distributors send out data feeds. OCLC's WorldCat is used by search engines as well. And online bookshops have their own staff who make manual changes to the data. Any of these sources could be making changes to your book listing, so tracking down the source of incorrect or incomplete data can sometimes be difficult. Your best approach is to make sure that your data's correct at Books in Print, and track the different places where your book is listed and correct data there as well.
For more information about book metadata and its uses, you can check out the following sources:
Embracing Data: A presentation at Author Revolution Day
A Self-Publishers Guide to Metadata for Books
Self-Publishing Basics: Introduction to Metadata