Users who subscribe to you will see your labels for relevant searches. As your labels become higher quality and more comprehensive, and as more users subscribe to you, your labels may start surfacing to more Google users than just those who explicitly subscribed. A number of factors help determine how broadly your labels appear -- such as the number of subscribers you have, how many different websites you've labeled, and, most importantly, how often your labels make it easier for users to find what they're looking for.
Great find! How long has this been out there? Watch someone come along and ask me where have I been? ;)
Developer guide - Pick your labels [google.com...]
Google Co-op gives you a way to improve search in the topics you know best. If you're a doctor, for instance, with specific expertise in a particular disease, you can contribute by using the labels in the health topic to annotate all the webpages that you know provide useful, reliable information about that disease. Your patients and other Google users could then subscribe to you and benefit from your expertise.