I don’t know the real answer. I like to pen down the main reason I can think of.
Let me first digress a bit. If you use LinkedIn, you would know that it’s possible for LinkedIn to create a profile of you based on the people you are connected to. This is in addition to all the personal details you provide about yourself. However, personal information like school and work will not completely distinguish two people. As the saying goes, “A Man is known by the Company he Keeps”, in addition to the personal information, the LinkedIn connections will give more information about a person.
The more accurate profile any company has about a person, the more it can target it’s services. For Google, that’s typically advertisement. With a service like Grand Central, Google will be able to amass the people relationships using the phone calls (A calls B). Currently, LinkedIn has no way to give weightage to a relationship. When two childhood buddies connect on LinkedIn that’s no different from when a recruiter hooks up with a person. Given that beyond that initial connection, the actual email communication happens outside LinkedIn, there is no better way for LinkedIn to establish additional weightage to each relationship.
On the other hand, the services offered by Grand Central allows it to track who is calling you all the time. The more calls you receive from a number, the more weightage can be given to that connection.
In addition, say you are trying to buy a house (well, now is not the right time to do so in many parts of the US at present, but say you are one of those who is still thinking of buying one). Now, if Grand Central figures out that you are working with some local real estate agent based on the calls you have been constantly receiving, Google can start showing you mortgage related ads, real estate ads etc. Ofcourse, they can do that based on what you are searching as well. But based on what it knows about that particular realtor, it can target even more.
Infact, Google has already been doing this with email. While Yahoo & Hotmail choose to not put any email address that you send an email into your address book by default, GMail does the opposite. It’s essentially cataloging all your network and the more you keep using Gmail, the more it can learn about you! By acquiring Grand Central, it not only knows your email network, it also knows about your phone network!