Personally, I own them because they make me smile. Plus they make everyone who fires my weapons smile and giggle at how absurdly quiet it is. This is especially true with my Osprey 45 using 9mm 158gr subsonics. But generally speaking, they are not as quiet as the movies portray them to be. My AAC M4-1k on the 10.5 AR is still loud.
On the serious side, I enjoy collecting firearms of different types, NFA stuff included.