They all seem to be copied from Chinese apps though their implementations suck more. Virtual currency, awards, and gacha style encouragement is very popular in Asian countries for communication apps. That's how they extract value because there are too many competitors and free apps.
Reddit might add shopping experience or real world items for purchase through coins soon. It's too obvious to not do that.
Thinking about it I'm a bit surprised that Reddit never built a marketplace product for subreddits. Looking back a few years I could see that actually being a pretty good product for them which would have been cool. You're definitely right that if they added it now it'd use their coins or do something utterly insane to make it terrible.
Reddit might add shopping experience or real world items for purchase through coins soon. It's too obvious to not do that.