Against various threats that range from deranged otaku trying to rush the stage to demonic invaders from hell that are trying to rush the stage because they’re also deranged otaku
Depending on which group you’re defending, doing well on the rhythm game portions can power up your agents, slow down fans, give you more money for paying defenders, or cause other useful effects
Bonus points if there's a goofy challenge mode where the idols are the security and the security are on stage performing, so they're not nearly as good at running interference.
towerssecurity agents and move old ones around between songs