Indeed, they are designed to reduce CPU load. Even so, if the game can't use multiple cores and previously was running game logic & OpenGL driver on the same core, now it's only the game logic. [1]
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[1] nVidia actually implements Vulkan on top of their OpenGL driver, but nVidia's drivers have been relatively well optimized in terms of CPU usage already. AMD is the bigger winner here.
The OpenGL threading model is completely screwed up. You essentially have a global lock per GL context, which means you're restricted to 1 thread for issuing rendering. And in GL a context is for everything, including shaders, texture states, you can't even reasonable do uploads of new scene data in a separate thread.
Vulkan fixes this big time, by allowing apps to construct GPU workloads for a single GPU in parallel. Only the final submission step (which is supposed to be very low overhead if the driver design is decent) is single-threaded per GPU context. And even for that Vulkan is better: It allows you to allocate different contexts for separate engines (e.g. rendering vs. compute vs. copy engine for data up/download to/from the GPU vram).
The lower CPU overhead is just the icing on the cake, the real deal is that Vulkan fixed the threading/locking model.
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[1] nVidia actually implements Vulkan on top of their OpenGL driver, but nVidia's drivers have been relatively well optimized in terms of CPU usage already. AMD is the bigger winner here.