The main purpose of the bundled APIs is to aid in the development of apps for dual-screen devices. On Surface Duo, optimizing for its screen is especially important because it doesn't have a flexible display. Instead, both screens have a bezel between them that could impact applications if not properly accounted for.
Microsoft has previously confirmed that all existing apps will work on the Duo, but their functionality will be limited without developer support. Apps that don't make use of the SDK will run on a single screen, rather than spanning across both, like the company showed with its Outlook demos. As you'll still be able to multi-task, the lack of spanning isn't a massive deal, but it could certainly help for some applications.
The included native Java APIs include DisplayMask, Hingle Angle Sensor, and more. Assumedly, the latter will let devs discern whether the device is folded back on itself and adjust the app to suit.
Alongside the Windows SDK, Microsoft introduced a Surface Duo emulator. With it, developers can see how their apps will behave on the device before it even releases.
As well as the device itself, the company is encouraging web developers to look at Chromium Edge, which contains specific features for dual-screen development. It plans to release its Windows 10X emulator on February 11th, with additions for its dual-screen Windows device, the Surface Neo.
The early announcement of the devices last year was primarily for releases like this. The hope is that by giving devs the tools early, the devices will launch with a significant suite of fully compatible apps. Whether developers will consider it worth the time investment remains to be seen.