Windows 8.1 with Bing is one piece of Microsoft's experimentation with how to monetize Windows. Microsoft already has thrown a lot of software and services in for free with Windows 8, so it needs to find ways to make money in a world where the pressure to drop OS licensing fees is increasing.
The Windows 8.1 with Bing SKU will be available only preloaded on OEM devices, Microsoft officials said. It's not available for purchase as a standalone SKU. Some of the devices running this SKU, "in particular tablets, will also come with Office or a one-year subscription to Office 365," according to Microsoft's blog post.