Instead of direct monetization, they probably measure by engagement. If a service has a permanent presence on your device, you get a LOT more engagement and recurring visits than a web version. Websites on mobile are one-time, visit-and-forget/move on things.

I'd love to be proven wrong though; what websites see daily, recurring, long time active usage?

Also not to be underestimated is that a mobile app will always have better UX than a website. Think of the subtle things like page transitions. Navigating Reddit still causes full-page reloads, whereas on the app it's a much more organic process. Speed is also an important factor.

They really hurt their own monetisation by making the subscriptions too expensive though. They more than doubled in price recently. It's not worth the price anymore for me.

It's also interesting to note that the old ui on a desktop does not have these useless messages you mentioned.