Bots aren’t just about curation. They can also create their own content. The Generate A Cat bot pairs the cat generation of G. P. Lackey (who has a Tumblr) with the bot-ness of Bronson Zgeb (who also has a Tumblr blog).
There are many procedural generator projects that exist on their own without a context like a game. Putting one in a bot gives it a space to exist where it can find an audience. A weakness of procedural generation is that if you consume the content quickly (such as if this was a desktop toy that generates cats) you’ll rapidly see the patterns and lose interest. By placing the generation function beyond your control, so that the bot chooses when to display a cat and not you, it maintains that interest longer.