The main benefit of Windows 10 on Xbox is that it will enable developers to publish Universal Windows Apps & mobile games to the Windows app store on phone, PC and Xbox with only minor UI adjustments required.
Yes, there was clearly some confusion on the Cagcast about what the "Windows 10" update for the XBox One would entail.
In the past year or so, Microsoft has been making a big push to expand their development environments. They recently open-sourced the C# programming language and .NET framework, technologies that had previously been closed, and very Windows-centric. (aside from community-driven solutions like Mono) Windows 10 has been a big part of this push. Microsoft has specifically made versions of Windows 10 that can be run on much more hardware platforms than previous versions of their operating system. It is now possible to install a version of Windows 10 called "Windows IoT" on a Raspberry Pi 2. (an affordable micro-computer)
Their Universal App System is a cornerstone of their new development initiative. It is a direct response to game engines like Unity, and web-application systems like Node-JS. When you develop an application in Visual Studio 2015 for their Universal App platform, it is possible to deploy that application to any number of different devices. Universal Apps will be able to run on the XBox One as well as desktop Windows PCs. They will also work natively with any Windows Phone. Moreover, the User Interface designer for these Universal Apps is designed to automatically scale based on what platform you are using them on. So the entire UI can be flexibly re-sized or re-configured based on what screen-size and input device is currently being used. In this way the application can be compiled once, yet run anywhere.
Naturally, there are some limitations. In order to compile a Universal App for iOS, you need a third-party library that is not free. This might change in the future, but it how Universal Apps are ported to iOS for the time being. It's actually quite remarkable that this is possible at all.
The draw of this initiative is for Microsoft to get as many developers as possible working in their development environment and using the programming languages and tools that they provide. For video game players, it may mean a lot more general-purpose software making its way to the XBox One. Of course, the on-line store for the XBox One is still going to be curated by Microsoft, so don't expect to see a bunch of non-gaming apps show up all of a sudden. Just because something CAN be ported to the XBox One with ease doesn't mean Microsoft thinks it SHOULD. And they are still the gate-keepers for the XBox One's on-line marketplace.