The whole Internet can read and contribute.
Useful for public FAQs like the Linux-on-PowerPC Faq-O-Matic.

The whole Internet can read, but only I can contribute.
Useful for maintaining searchable documentation, like this site.

The whole Internet can read, but only a certain list can post.
The whole Internet can read, but only those in my domain can post.
Useful for help-desk applications.

Only users in my domain can read it.
Useful for when the information shouldn't leave your company or department.