While one can come up with many possible verbal explanations for why Google might want to go this direction, there is some powerful math that lies at the heart of it: supermodularity. Here is the definition:
is supermodular if
for all x, y Rk, where x y denotes the componentwise maximum and x y the componentwise minimum of x and y.
A firm such as Google for which the production function relies almost exclusively on information (yes, there are servers and people as well) will exhibit super modularity almost by definition. Why? Because if X and Y are different information vectors, then as long as they carry some joint signal, the inequality will be met as you can always choose to discard additional information (meaning you always have access to the component wise minimum). In plain English: if you have access to both the search history (X) and the social graph (Y) of a user, you can always “do better” than two separate services that only have access to one of these respectively.