Because it's (well, at least 6 months ago was, now it could have improved) the only solution to reuse the GPU without rebooting X.

With a practical example:

1 - You boot up your Linux base system.

2 - You fire up KVM with GPU PCI passthrough with a Windows guest to play some games.

3 - You shutdown windows.

Now you can't use the GPU anymore, the module was assigned by KVM and you can't - for instance - run a CUDA simulation in the Linux host. You need to reboot X (you don't need to reboot the system).

The workaround/solution is to fire up KVM with GPU PCI passthrough to another guest (This time a Linux guest) and in there you will have full access to the GPU to do CUDA computations (or whatever else you want).