Primary use cases:

(A) Development and testing in multiple environments (to fit 99.99% users' expectations) <-- as many different VMs with different allocated disk & memory & other resources as possible; as many different OSes as possible, including Windows XP/7/8/8.1/10/10CU1703, OSX, Android and whatnot;

(B) 3D rendering (Blender), CAD modelling (Solidworks are Windows exclusive, whilst Autodesk 3D supports also Mac/OSX), DaVinchi Resolve/Fusion (RedHat/CentOS native + Win/Mac) in multiple VMs (for faster delivery of the final product) <-- dGPU passthrough, to give 1 dGPU to each VM, in order to render 3x times faster (multi-machine rendering within 1 rig; X399 has 4 PCIe slots meaning 3 for the passthrough to VMs and 1 for the host, as AMD has no onboard graphics card in X399 so far, so it could be used for Blender which is native in Linux;

(C) Adobe AE,PR to render/edit movies using pre-made modules and 3D models from (B); as GPU is irrelevant in both AE and PR (only sued to speed up previews) so no need for dGPU passthrough for these particular tasks;

(D) Developing and testing external embedded devices using native SDKs which require direct native unvirtualized USB access (read: Amazon Echo*, Oculus Rift, Leap Motion, own private proprietary developments, including your beloved ECOVACS DEEBOT N79 Robotic Vacuum Cleaner and whatnot...) <-- i.e. PCIe passthrough (for a PCIe USB card), as all on-board USBs are grouped in IOMMU

(E) While doing the above - running several (or tens of) "small" local staging servers, firewalls and whatnot - all in separate VMs - to test before synching with the shared staging and/or production servers;

(F) Some graphic editors do not work in Linux, so to import some legacy developments, I need a Win/Mac [virtual] machine.

(G) Games? I barely have time to sleep after the above, so not so much. Do not remember the last time I had to think about gaming.

You are welcome.