I know what a yaw damper does in  real life, but what does it do in FS9?

Several FS9 jets have a yaw damper switch (usually on the autopilot), but I cannot detect any difference in flight characteristics with the switch on or off.

Is it just cosmetic or does it really do something that I just cannot detect ?

I've even checked the ".air" file to make sure it is enabled.

Thanks,