Wire-framing isn't a new concept and prototyping tools have been around for some time. But they weren't fast enough to enable real-time collaboration of ideas during analysis meetings. So you could not use wire-framing as a substitute to written specifications.
But now with VisualSpec you can. Imagine wire-framing the entire project, and changing your mind as many times as you please, without paying a big bill for it. Try different ideas, play with them and get everything right before risking your development budget.