Google Mobile's page for Android has an interesting message: "The best Google Mobile applications are available for phones that run the Android operating system". That may seem obvious, considering that Google developed a lot of applications only for Android: Gmail, Google Talk, Google Finance, Google Goggles, Google Maps Navigation, Google Shopper and more.
Google's applications are one of the most important competitive advantage for Android, but that doesn't mean that some of the applications couldn't run on other platforms. If Google didn't develop Android, many of the applications would be available in Apple's App Store. Google claims that it takes "great care to ensure that [its tools] ultimately serve you, rather than [its] own internal goal or bottom line", but restricting innovative tools to a single platform doesn't benefit users.