Harry Potter book first edition for sale

Harry Potter and the Philosopher’s Stone (First Edition) at Bonhams’ auction

ROWLING, J.K. (B. 1965) Harry Potter and the Philosopher’s Stone. London: Bloomsbury, [1997]. Why is it so rare? It is a first edition (1997), plus it has a rare hardcover that was produced in very limited numbers (around 500 copies). Moreover, it is signed by J.K. Rowling. Seller: Bonhams Link to the auction: https://www.bonhams.com/auctions/26890/lot/137/?category=list&length=12&page=1