I figured i better do a google search
http://en.wikipedia.org/wiki/Steering_damperit is probably also helping with the bumpsteer and the shocks transmitted thru the steering linkage (like Bounty said)
but my gut feeling still tells me that it was fitted from factory after their road tests where they probably discovered some resonance issues - vehicle manufacturers often come up with little bandaids once they get the vehicle on road tests - common one was when they started fitting molded dashboards and at certain speeds the whole thing would start vibrating out of control, you will find on some older vehicles some bolted brackets where you would think there shouldn't be one. With the steering linkage is probably related to the tire diameter and the length of the bars, since the tire is always changing diameter due to temperature, pressure and wear there was no point in changing the length of the linkage (not to mention that would have involved changing the steering knuckles as well) - so the easiest and cheapest solution would be to put a shock absorber on to cut out the auto induced waves in the assembly.
again, my personal opinion so could be off...
EDIT: should probably explain a bit more what i mean about resonance
if you have a linear part between 2 solid points and the wavelength of the vibration induced in that part would divide the total length of the part to an exact number or integer if you wish (the length of the part is an exact multiple of the wavelength or in other words the length of the part between the points where it is bolted on is divisible by the wavelength of the vibration) then the amplitude of the vibration will amplify by the reflected wave when it reaches the assembly point (where the part is bolted on) - the new reflected wave and the initial wave will then vibrate at unison amplifying itself, reaching the other end would result in the same thing, the amplitude will be again increased and the process goes on and on until the catastrophic failure of the whole assembly - the only way to prevent it is to not reach that vibration point, change the length of the part so it doesn't match the wavelength (this is hard since the same thing can and will happen at different frequencies) or to install a damper on the part (the steering stabilizer in this case) - what that will do is to turn the oscillation into an attenuated one where the amplitude is cut out by the damper preventing the self multiplication of the vibration and cutting out the resonance effect.
this is a phenomenon that has led to failure in some bridges for example where the tiny up/down movements of the deck has amplified to the point where the bridge has collapsed. In the case of a vehicle you can simply reduce the speed or accelerate to pass that resonating frequency.