GLX and EGL have two roles, one is the WSI part, the other is context creation and device enumeration which Vulkan had the good sense to add to the core standard.
That's freaking awesome. I had no idea, is this a recent addition? I've read SDL source code some years ago and I remember seeing a X11 implementation only.
It’s a spec for how GPU APIs can share data without necessary copying and converting through CPU memory. Mainly because the OpenGL specification explicitly does not define anything to do with windowing. But, EGL is not just and OpenGL - XWindows bridge. CUDA, GStreamer and several other APIs use it to communicate.
EGL is primarily about windowing system integration, it's what lets you initialize an OpenGL context in the first place.
It doesn't really accomplish interop between different APIs on its own, though of course various interop methods are based on it — e.g. if you want to import a dmabuf, you'd use EGL_EXT_image_dma_buf_import. But, say, on the Vulkan side you'd use VK_EXT_external_memory_dma_buf — there is no EGL with Vulkan, Vulkan has its own WSI subsystem.
