I’d assume they are to regulate how often the scenarios happen. A “common” scenario would have higher priority than a “rare” one. Seems logical, doesn’t it?
But who knows besides Radiant.
Rarity in immigration… It might mean that you’d get more valuable citizens or more loot, depending on the rarity… But as Avarian put it, only RadEnt knows!
The rarity of a scenario, at the moment, determines how often the scenario spawns. Rarity within the immigration scenario determines how rare a given type of immigrant is.
The scenario metadata may change quite a bit in the next few months, though, but you’re welcome to play around with it till it does.