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Paid subreddits. I mean look at the free to play live service games, how people like to buy stuff that not everyone has. We can laugh as much as we want and think how dumb this is. But people are dumber. It will drive away some people from Reddit, most won’t care and just stay using the “Free” stuff. Some will pay for the subreddits.
Maybe NSFW content or support subreddits for paying customers.
Given enough money, everything in the market is for sale. This is just a tactic to make the value higher.