If your horse works on hard surfaces such as hard and dry ground or paved streets, eventually this surface hardness can lead to musculoskeletal damage.  Ideally, horses would be maintained on a variety of surfaces with minimal hard surface exercise but in some situations, this is not possible.  Dr. Amy Barstow indicates at thehorse.com that this is the case with police horses or even leisure horses when there is a severe drought and the ground is very hard. 

Dr. Barstow recently investigated a pour-in polyurethane sole packing material to help reduce sole trauma and found it helps reduce certain forms of vibration and forces in the hoof.  The theory is that when polyurethane absorbs the shock from the hoof impacting the ground, it prevents the shock from traveling further up the leg where it could cause wear and tear injuries.  The researchers tested five sound horses in four shoeing conditions with standard steel shoes and standard aluminum shoes, both with and without sole packing.  They used an accelerometer to measure vibration and forces as each horse trotted across hard ground.  Results indicated that impact was 30% less in the horses with sole packing regardless of the type shoe used, but total vibration was less in the aluminum shoes with packing compared to the steel shoes.  So, the sole packing made a big difference in the vibration transmitted up the limb. 

However, this does not indicate every horse on hard ground should have sole packing.  Also, realize this was a controlled study and problems can occur with sole packing in the real world because if the horse stands around the water trough, moisture can eventually get under the packing and can cause the sole to soften and loss of sole, so work with your vet and farrier closely to see if sole packing is the best for your horse.