I've been somewhat curious about the yaw-dampers that I see featured on certain payware twins for FSX, specifically the Beech Duke and the Piper Cheyenne.  Aside from being a prerequisite for Autopilot activation, they don't seem to do anything noticeable.

Since straight winged aircraft don't suffer from Dutch Roll, what is the purpose of the YD?   Automatic turn coordination?  The RA Beech Duke in the sim doesn't appear to have any form of auto-coordination whether the Yaw Damper is on or off.  The Cheyenne by contrast, seems to be auto-coordinated even when the YD is Off (and consequently "feels" somewhat unrealistic, as if it is riding on rails, completely devoid of inertia, with no tendency to slip or skid)

So in the real world...what are the Yaw Dampers doing on light twin GA craft, and what role do they play in auto-coordination?  I gather that the presence of a Yaw Damper does not always imply auto coordination?  Are the aforementioned sim planes true to their real world counterparts?

It can be very important with twins, for two reasons:
First, since the throttles can be controlled independently, there's always a chance that there may be more thrust on one side than the other, for brief periods.
Despite the straight wing, it will yaw (most likely to the outside of the turn) when a roll is initiated without rudder input. Obviously, if there is a partial or total loss of power with one engine, especially right after takeoff, immediate rudder input is needed. Having an "autopilot" for the rudder (constantly trying to keep the "ball' centered) helps with that, as it can take precious seconds just to identify the ailing engine and feather the prop.

Second: most twins have props which rotate in the same direction, so in high-power/low airspeed situations, such as takeoff, the plane will tend to yaw quite a bit due to p-factor, torque, etc. So a yaw damper lends a helping hand- or rather, foot- in that situation.

"Dutch roll" can be initiated in just about any airplane, although some are more prone to it than others. You can get the wings out of level by using only rudder, and you can get the nose to swing by using only aileron.

In flight sim you almost always have both engines running identical.  In real life, it is almost impossible to have perfect matched engines on a twin, one always runs just a little different.  Even if you do, it would be very easy to not get the throttle settings 100% identical at any given time, hence the use of a yaw damper, which is just basically the third axis of an autopilot.