Sound is energy. When sound reflects from hard wall surfaces, the results can be very uncomfortable. Sound needs control.
Sound needs to have a certain degree of both absorption and reflection.
Sound absorption is a ratio of energy which strikes a surface, less the amount of energy which is reflected back from the surface. A common measurement term used in the industry is the NRC rating which stands for Noise Reduction Coefficient. It is the average of specific frequencies as absorbed by certain materials. Compare this to an acoustically treated wall with an NCR of .75 achievable with fiberglass or Dacron covered in an acoustically appropriate fabric. Ask any teenager's parent to describe the alternative.