I just wanted to chime in that shooting a 9mm suppressor with sub sonic is quiet enough for me to not have my ears ring. It's even quieter when fired wet.What I do wonder is why there's so much interest in them. They make the host firearms longer and bulkier, and don't negate the need for hearing protection, especially if you're firing supersonic loads.
With supersonic loads dry it sounds like an unsuppressed 22lr which is still quieter on the ears than firing supersonic 9mm without a suppressor.
As to why? Why not? They do help with noise and to not upset the neighbors who may or may not hate the sound of gunfire.