Imagine you own a store. Every week 2 people come in. They talk and shop for a long time. You spend a lot of money showing them all the products and answering their questions. One of the two spends always buys a ton of stuff and the other one never buys anything. You realize you can save a lot of time and money by banning the other person since they never buy anything. Next week, neither of them come in.
Thats reddit. The people that use the first party app and click ads and make money do it because of the content created by both the first party and third party app users.