Complements are, in economic terms, any products or services that tend to be purchased or consumed together, such as hot dogs and mustard or lamps and lightbulbs. For Google, everything that happens on the Internet is a complement to its main business..
...It's this natural drive to reduce the cost of complements that, more than anything else, explains Google's business strategy. Nearly everything the company does is aimed at reducing the cost and expanding the scope of Internet use. Google want information to be free because, as the cost of information falls, we all spend more time looking at computers screens and its profits go up.
From The Shallows, an explanation of Google's many ventures beyond search and advertising that makes sense in terms of its broadband initiative.