Agent support allows better integration with the guest. For example, it allows copy and paste between the guest and the host OSes, dynamic resolution changes when the client window is resized/fullscreened, file transfers through drag and drop, ...
The agent is a daemon/service running in the guest OS so it must be installed if it was not installed by default during the guest OS installation. It also relies on a virtio-serial PCI device and a dedicated spicevmc char device to achieve communication between the guest and the host. These devices must be added to the virtual machine if we want to agent to work properly in the guest.
The needed devices can be added from the virtual machine details. Click on "Add hardware" and then add a "Channel" device with type "Spice agent (spicevmc)". This will automatically add the needed virtio-serial device in addition to the spicevmc channel.
Two distinct devices must be added:
<devices>
<controller type='virtio-serial' index='0'/>
<channel type='spicevmc'>
<target type='virtio' name='com.redhat.spice.0'/>
</channel>
</devices>