I think it has a few features that make it preferable over just sending e-mails back and forth...
Multiple writers traveling together? Use Writeboard (
http://writeboard.com/) to collaborate vacation plans and/or
writing ideas. Create a virtual whiteboard at the site and add as
many collaborators as needed. The Writeboard will automatically
update when new notes are added, and you can export all the content
into an .html or .txt file. Also, the site requires that you set a
password for your Writeboard, so you don't have to worry that your
top secret info will be compromised!