Actually the proper term for shock absorber and as used by the Brits is DAMPER and its purpose is to damp the oscillations on a spring supported system.
The rubber suspension tends to damp the oscillations (turning the excess movement into heat), but the damping is not complete. Ideally the shocks would be smart enough to have variable damping to change their effect for sharp jolts, high displacement regular frequency etc. as necessary and the new, better valved shocks and electronic controlled for cars do this so you get a smooth ride on smooth roads and higher damping for rough roads.
Shocks are valved to have a greater effect on jounce and rebound, since the initial movement needs to be undamped (to some extent) and the rebound is damped. This means that the first part of the bump moves the wheel and the rebound is slowed. The shock valving does this. Shocks are rated as to 50-50, 20-80 etc depending on what the chassis tuner is looking for.
What we are looking for the the carryover oscillations to be damped so that the trailer is not bouncing all over and shaking the contents more than necessary.
AL-KO in the US contends and the other torsion axle manufacturers say that the damping of the rubber is enough for a trailer.. AL-KO Europe adds shocks and the Euro axle is setup to add the shocks to the axle and their trailer frames. Viewing their video shows the usefulness clearly. The benefit is in smoother ride and damping the tendency to sway from side to side.
The enemy of towing a trailer is the tendency of the combiner tow vehicle and trailer to enter into undamped oscillations and the amplitude of such increase instead of being damped and decreasing. Not only do the shock absorbers create a smoother damped ride, but also damp the side to side oscillation from the CG of the trailer being higher than the roll center.
All in all a Win - Win for shock absorbers. Perhaps the fitment of an anti roll bar would complement the suspension of trailers as well for the same reason?
This damping of the oscillations is the reason I think that my VW Sportwagen tows so well. The IRS and the fairly stiff shocks damp the oscillations better than a softly sprung SUV. The result is what is called Dead beat damping where displacement is quickly damped out and swaying is minimized. Shocks on the trailer could only help!
The addition of an anti sway bar to the trailer to TV combination adds the friction damping to help this sway and all it does is change the energy to heat like any other damping system. in the European AL-KO setup the actual coupling has friction pads that are clamped to the ball with a lever to do the same thing and the Hitch balls do not mount with a threaded shaft and nut that would be loosened for this reason.
Since Al-KO basically invented the rubber sprung torsion axle they probably have a fairly good idea as to the best way to operate one as well.