While most of Google's data is already encrypted, Google's utilization of HSTS goes a step further by preventing users from mistakenly heading to HTTP URLs by converting potentially unsafe HTTP URLs into more secure HTTPS URLs. For instance, you might accidentally type in a URL without protocols and find yourself redirected to an unsafe destination. HSTS help curb those issues, especially among less internet-savvy users.
Google is looking to deploy the changes as soon as possible, but there's still some additional work to be done before it's ready to go. HSTS is now active for Google's domain, however, in the meantime. It will be extended to additional domains and Google products soon.