The big picture is probably that GB10 is destined to show up in laptops, but NVIDIA couldn't be bothered to do all the boring work of building the rest of the SoC and Mediatek was the cheapest and easiest partner available. It'll eventually be followed by an Intel SoC with NVIDIA providing the GPU chiplet, but in the meantime the Mediatek CPU solution is good enough.
From NVIDIA's perspective, they need an answer to the growing segment of SoCs with decent sized GPUs and unified memory; their existing solutions at the far end of a PCIe link with a small pool of their own memory just can't work for some important use cases, and providing GPU chiplets to be integrated into other SoCs is how they avoid losing ground in these markets without the expense of building their own full consumer hardware platform and going to war with all of Apple, Intel, AMD, Qualcomm.
From NVIDIA's perspective, they need an answer to the growing segment of SoCs with decent sized GPUs and unified memory; their existing solutions at the far end of a PCIe link with a small pool of their own memory just can't work for some important use cases, and providing GPU chiplets to be integrated into other SoCs is how they avoid losing ground in these markets without the expense of building their own full consumer hardware platform and going to war with all of Apple, Intel, AMD, Qualcomm.