Aren't earplugs way cheaper, lighter, and more effective?
The reason for a suppressor is so that OTHER people don't hear it. Let's not beat around the bush here.
So, other than assassination, and night time poaching/hunting (because guns are typically more reliable than snares or archery), why would one need that?
JD brought up a rare, but still kind of silly reason... as there are generally noise ordnances based on time of day.