0.0.0.0 is simply a way of saying all IP's. This way, it listens on 127.0.0.1, your normal IP, and any other. If it binds to a specific IP, then it binds to that IP only, and wouldn't appear on the others.
For security reasons, some daemons bind only to 127.0.0.1 so they can provide local services, but aren't available to the outside.