Native support for audio and video codecs is helpful for a platform. Software will be able to call upon the operating system's built-in functions, rather than integrating a solution. Of course, some will continue to roll their own, and that's fine, but it is obviously helpful for the foundation to have its own solution (especially in cases where licenses and royalties are required).
Windows 10 is expected to increase its platform support to include FLAC, MKV, and HEVC (h.265), and more may be coming. The tweet from Gabriel Aul suggests that this will be available starting in the next preview build, which will land in early 2015. Hopefully these additions include both encoding and decoding support, possibly allowing audio and video editors to take advantage of these formats.
The next build of Windows 10 Technical Preview is expected for early next year. The full OS is said to launch late that year.