> Perhaps you could go further and tease out why...

I suggested it for much more mundane than you suggested: Google is likely to have valuable political, economic and political information because politicians, businessmen and service members have personal Google accounts, and employees are the weakest link in an organization's security. I don't think anyone's going to be sending out the designs for the next stealth fighter over Gmail, but plenty of people would be sending out innocuous documents, contact information, personal problems, etc. Intelligence services thrive on innocuous information in aggregate.