The Windows 10 Anniversary update, due later this summer, represents a major landmark for Microsoft. As well as being a significant update for Windows 10 on the desktop and Windows 10 Mobile on phones, the release is also coming to the Xbox One. For the first time, the Xbox One will be running essentially the same operating system as desktop Windows. Critically, it will also be able to run many of the same applications as desktop Windows.
In a lot of ways, this represents the realization of a vision that Microsoft has been promoting for more than 20 years: Windows Everywhere. Always important to Microsoft's ambitions for Windows as a platform, the Windows Everywhere ideal has a renewed significance with Windows 10 and CEO Satya Nadella's promise that Windows 10 will have one billion users within the first three years of its availability. The purpose of that promise is to send a message to developers that Windows is a big platform, a platform that they should still think about and create software for.
But if it is to have a hope of hitting that one billion target, Microsoft needs more than just PC users to get on board, which makes it important for Windows to run on more than just PCs. Hence the need for Windows Everywhere.