On Windows, we really only have one graphics driver per GPU. On Linux, however, there is a choice between open drivers and closed, binary-only blobs. Open drivers allow users to perpetuate support, for either really old hardware or pre-release software, without needing the GPU vendor to step in. It can also be better for security, because open-source software can be audited, which is better (albeit how much better is up for debate) than just having a few eyes on it… if any at all.
As we reported a few months ago, AMD has been shifting their structure. Rather than two completely different code-bases, AMDGPU is an open-source driver, officially supported by AMD, that communicates with the Linux kernel. This chunk is compliant with the GPL, so it can be bundled with the operating system. Above this, a user space driver adds the various APIs, game-specific optimizations, and so forth. AMD calls this plug-in component AMD GPU-PRO.
This component has now been released for Ubuntu 16.04, which includes OpenGL 4.5, OpenCL 1.2, and Vulkan 1.0.
Open-source developers can create their own components, using the same AMDGPU hooks that AMD uses, and release those on their own. This is not a perfect solution, though. If, at any point, AMD disagrees with a necessary, proposed change, then the only way forward could be to fork the project, which AMD wouldn't support with their closed-source blob, leading to the previous situation. That said, AMD is putting a lot of effort into this, so it would stand to reason that they aren't intending to throw all of that away over a pull request.
Either way, you can get AMD GPU-PRO Beta from AMD's page for Ubuntu 16.04. SteamOS added AMD GPU-PRO with their 2.80 update last week.