Requirements people, designers, and developers think on different levels. When they meet to plan what needs to be done they focus on different aspects of the specifications rather than on the problem that needs to be solved.
Working interactively on the page turns it into a shared design problem. Everyone in the team can participate in naming items, and the business analyst can remind the team of the real-world terms. Once the prototype takes shape, testers can apply their tests to the paper: How many characters? Is this required? Where do you go when you click on this?