The QEMU Guest Agent is a service that runs inside the guest virtual machine (VM) and communicates with the QEMU hypervisor. This is different than the VirtIO drivers. Its job when you check the QEMU agent box is to allow the host to issue commands to the guest, like filesystem freeze and thaw, shutting down the guest cleanly, and it also helps provide more detailed guest information. This is really useful as well when snapshotting VMs or live migration activities.