I think another major reason is that if someone is seeking to amuse themselves then in a browser they're as likely to skip over to another website; whilst in an app there's a mental barrier to that. I imagine people are more likely to change and view another subreddit, rather than jump to another website altogether.
There's a censorship angle too, wherein the app operators get greater control over the message their users receive than do website operators.
I think another major reason is that if someone is seeking to amuse themselves then in a browser they're as likely to skip over to another website; whilst in an app there's a mental barrier to that. I imagine people are more likely to change and view another subreddit, rather than jump to another website altogether.
There's a censorship angle too, wherein the app operators get greater control over the message their users receive than do website operators.