PCI Passthrough is most commonly used to pass a dedicated video card from a Linux host to a Windows guest.
This way, the Windows virtual machine can handle any graphically-intensive content, like games, rendering- and media editing software, at close-to-baremetal performance.
Many people want to switch to Linux as their main operating system, but don’t want to give up their games or other Windows-only software.
With PCI passthrough you can run Linux as your main desktop and fire up your Windows VM whenever you need any Windows-only software.
PCI Passthrough can also be used to pass other devices, like USB controllers, PCIe SSDs and network cards. You can even use it to pass an entire SATA controller.
This way you can, for instance, set up a PFSense VM on Linux, pass a dedicated network card to it and use it to set up a high-performance low-energy router.