For example I use onion2k@myemailprovider.com to send emails to alice@gmail.com, bob@gmail.com and charlie@gmail.com. Google now have a profile for onion2k@myemailprovider.com with a graph that includes nodes for alice@, bob@ and charlie@ and edges for the relationships between the four people, and links to anything we've discussed. If david@gmail.com then sends an email to onion2k@myemailprovider.com Google know that david@ has a second order link to alice@, bob@ and charlie@ despite the fact that they have never communicated or informed Google of this relationship, because Google have a profile based on onion2k@myemailprovider.com - an address that Google might not have a justifiable reason to be building a profile on.
Whether or not you believe that is a reasonable use of data is up to you. Some people think it's not.