Because spammers can’t easily obtain new IP addresses through legitimate means, they frequently resort to stealing IP address blocks that are dormant and aren’t being utilized by the rightful owners. There is a thriving black market in IP addresses; spammers don’t care whether the source of their IP addresses is legitimate or even legal. A cybercriminal that can steal a large IP address block (for example, a /16 or 65,536 IP addresses) can generate thousands of dollars per month.

Source: Verizon Routing Millions of IP Addresses for Cybercrime Gangs