It’s similar on android. Most apps that use it legitimately relate to health or fitness. I suspect that your headphone example would apply to Pixel headphones, also.
Meta probably wants it mostly for advertising purposes. They aren’t exactly cautious when selling data, though, so who knows?
I’m not sure. Normally, most users would come back as you describe. But if the lack of mods gets too serious, then most users will begin to get bored or annoyed. If other platforms scale up well, boredom translates into “I heard about…”