Give me your money or the computer gets it.
Turning hijacked computers into cash is still hard work for most computer criminals. They've got to trick the infected PC into sending spam, then trick a recipient into buying a useless product -- or they have to steal online banking passwords, log onto a victim's account, bypass the bank's money transfer fraud controls, and so on.
It's much easier to just demand cash directly from infected users -- a crime that's the Internet's equivalent of kidnapping. "Give me all your money or your computer gets it-" is the basic proposition.