Long trailers will have more deflection mid-span than short ones causing more bounce because the suspension and tires are not ridgid support points. By adding the mid-span stabilizer you make a long beam 2 short ones and significantly reduce the bounce.
This is a cool DIY solution that essentially the same as adding a 3rd set of stabilizers or a couple of jacks just ahead of the axles. People often have this stuff lying around the house so cost would be very little. Thanks for the contribution!