Because you get the PDF immediately and it's cheaper for you. For example, suppose you wanted a softcover copy. You could pledge at $20; you immediately get the PDF, and then when the book comes out, you get a coupon that lets you buy it for about $6+shipping. Conversely, if you just waited for retail, you'd be paying $35+shipping. The same relation applies to the hardback, so you save about $10 off retail by pledging.