It is an attempt to pre-populate new instances with some popular communities which is seen as a way to improve discoverability. I find the general concept of using "popularity" for that to be somewhat problematic, but the main issue I have with the actual implementation is that it uses lemmy.ml as the source of truth for that, and there is no way to change that*. I have since learned that it is possible to entirely turn off that new feature and thus have the current behavior, but then you basically have the choice between two bad options and it feels like malicious compliance to a request to improve discoverability.

*the dev explained to me on the Matrix chat that there are some technical reasons for that, but it reeks like a convenient shortcut to a difficult problem, that causes more harm than good.