How the Emotet botnet flooded inboxes across Japan
Emotet has evolved into a monetized botnet-as-a-service platform over time, designed to deploy other malware or run campaigns via pay-per-install (PPI) offers, where cybercriminals pay other cybercriminals with a botnet network of infected devices to spread their malware to the already infected devices. Therefore, Emotet's authors strive for the largest possible Emotet botnet.
Source: securityboulevard.com