The villains from both games are invaders from another dimension who need to extract mana from humans to sustain themselves.
Since these are porn games, the best way to harvest said mana is also the fun way - fucking women.
The presence of a magical girl results in a high risk, high reward scenario, since magical girls are extremely dangerous but also worth vastly more mana than normal humans. IIRC, Noble Rose is called out as possessing more mana than all of the muggle girls in her city put together.
Magical girls work a bit differently in the two games, though - since in Listeria they're scouted by familiars/mascots who come from the same dimension as the invaders (though I am amused that in Listeria, there's a single throwaway line tying magical potential directly to boob size), while in Noble Rose our heroine just spontaneously manifests as a magical girl once under sufficient stress (her and her best friend getting cornered by the rape monsters).