Market share analysis is a handy tool for a couple of reasons. First, it allows publishers to discuss competitors' sales figures with an account without giving away confidential point-of-sale information (e.g., "your life-to-date market share for this title is only 16.2%, compared with Competitor A's 22.8%"). To be clear, I'm not using "Competitor A" in my usual tongue-in-cheek way; we actually don't reveal the names of the competitors whose sales data we're citing, simply because the account in question clearly has access to their own sales data and could reverse engineer the competitor's sales data from those numbers if they were so inclined. (For example, if I were to tell Barnes & Noble that Borders had 20% market share against their 15% for a given title, Barnes & Noble could simply look up the number of copies they sold of that title and calculate how many Borders must have sold.)
Second, market share analysis provides context for the discussion of a title's sales record. Saying "Chain A sold 4,282 copies of Boy Wizard and the Impossible Task" at a publisher meeting or telling Chain A that they sold 1,421 copies of Comp Title One during a sales call isn't very effective if you don't provide any data that illustrate the size of the overall market. Those numbers might be great if they constitute 30% market share, but disastrous if they work out to 4%. You get the idea.
Third, trends in market share allow publishers to see how various accounts are selling (either overall or by format, genre, &c) over any given interval of time. Is National Account A losing market share to National Account B overall? What about just hardcovers? Are both A and B staying roughly even, but losing proportional amounts of market share to Wholesale Club C? Is this largely due to a few titles or an entire format (e.g. mass market paperbacks)? The list goes on.
What does this mean for you, gentle authors? Well, not a lot, to be honest—market share discussions are much more salient for publishers and retailers than they are for individual authors. That said, it might be helpful for you during the publication process to think about how and where your book might be sold: do you see it as a grassroots-type literary epic that will take hold by word-of-mouth and primarily sell at independent bookstores? A mass market paperback paranormal romance that will sell thousands in the $8 rack at Walmart? A cyberthriller that will take off on Amazon? There's a different sales model for each, and the way the market divides those sales helps determine how your book will do and how your career as an author will evolve.