I’ve heard a fairytale about a handful of hackers in the world that have huge numbers of computers infected. The malware they use is unobtrusive and waiting to be activated, turning into sleeper agents of the hacker. There would be no point for one person in checking all these computers by hand, but being able to control all of them at once they become commander of an army. The only reasons I can think of for such an army is blackmailing, or d-dossing some of the internet backbones to blackout the internet in parts of the world (if that’s still possible).
I’m very skeptic of the story though, as I find no satisfaction in the explanation of what they use those armies for. But it does sound very plausible to infect a massive number of computers. This can be done quite easily, we’re not all running Windows 7 and OS:X. A considerable amount of computers in the world still run on pirated and outdated operating systems like windows 95 (think Asia, Africa, etc).
A more serious answer would be: That it’s not only scriptkiddies that want to get in the ‘scene’ though, a lot of them do it to for career opportunities. Not only do they need the experience, but also the exposure to get noticed by companies that need to protect themselves or their users. Plus if there were no hackers, there would be no jobs for them. It’s the circle of a hackers life.