The idea behind this is to have the ability to create users for sub sites on your WPMU install.
For example, I had to manually create a function, unless you know of something that can do this, that on sign up on the main site, I would create a user for my support sub site. This way they get immediate access to support through the sign up process.
The function would ask pretty much do wp_create_user and add_user_to_blog in on function asking the username, password, email, role, blogid.