There's a huge number of synergies that this deal provides.
(a) NVIDIA becomes a full-fledged full-stack house, they have both CPU and GPU now. They can now compete with AMD and Intel on equal terms. That has huge implications in the datacenter.
(b) GeForce becomes the reference implementation of the GPU, ARM processors now directly fund NVIDIA's desktop/datacenter R&D in the same way consoles and Samsung SOCs fund Radeon's R&D. CUDA can be used anywhere on any platform easily.
(c) Acqui-hire for CPU development talent. NVIDIA's efforts in this area have not been very good to date. Now they have an entire team that is experienced in developing ARM and can aim the direction of development where they want.
Basically there's a reason that NVIDIA was willing to pay more than anyone else for this property. And (Softbank's) Son desperately needed a big financial win to show his investors that he's not a fucking idiot for paying $32b for ARM and to make up for his other recent losses.
