I’m more familiar with Vulkan, haven’t worked with DirectX 12 myself. But I believe like Vulkan the big win is it allows developers to schedule draw calls from multiple threads so that the CPU work of passing draw commands to the graphics card can run much faster (across more than one thread). But the game engine needs to be modified to take advantage of this, it’s not a free win.
Other than that, just generally a newer and more efficient library so you’d see some performance improvements there of course, and access to newer APIs such as ray tracing.