Its to make the community appear active at a glance. People are far less likely to join a community that has zero post activity since a large number of people lurk and the most surefire way to simulate post activity and ensure that the content is appealing is to mirror popular and active posts from already popular communities.
It's a bit like that thing where scientists put out fake decoy puffins to encourage other puffins to land on an island. Or how new cafes and coffee shops will sometimes hire or otherwise incentivise people to stay there all day so the cafe is never empty while they're still trying to drum up actual traffic
That'd actually very similar to how trig points work! It's a giant orbiting trig point. Someone call the ordnance survey!