The information you provide under your health records is used for your own personal purposes, and can be printed and taken with you to a doctor's appointment. If an office is integrated with Google health, then the information is already accessible to them In terms of the extended health services provided through Google's health portal, registering for any of these, and linking it to your Google health profile provides you with automated updates to that third party service.
The doctor search is quite similar to what you'll find through a regular Google search, except this is a specialized search tool. There are no filtering options, however, so going through a page of results can b a cumbersome task. Once you find a doctor you like, however, they can be linked to your Google Health profile as well.
With the current offering, the most powerful aspect of Google Health is likely its consumer-facing portal, which a number of third-party services will be interested in having a spot. This appears to be more integrated with third parties than Microsoft's HealthVault. Even though Google is explicitly not in partnership with these third party services, having a select number of these services is almost acting as a recommendation to the end user.