For a few reasons, really.
WebProof splits projects into pages, and deals with each page separately. As soon as a step is performed on one page, it is off to the next step.
Nobody has to wait for the entire document before they can get at it.
WebProof also notifies each person when they have a page to work on. If you're not logged in, WebProof can send an email. If you're away from your desk (or out of town, or out of the country), WebProof can send an SMS text message. WebProof stamps every Page Status change with date and time. It becomes clearly obvious when someone delays addressing a page for a long time after it was ready. You'll be surprised how fast response time gets. We also observe that nearly 50% of Sticky Notes are created, not from 9-5 but between 6pm and midnight. What you normally would receive sometime later in the day, you now get first thing in the morning!
Also read the article "Six reasons you should be using WebProof for your projects".