Shortly put, botnets are extremely appealing infrastructures mostly because of their tremendous computing power simultaneously distributed on millions of machines. Recent estimations claim that a medium- or large-sized botnet is able to provide more computing power than the most powerful supercomputer on Earth.
The most powerful supercomputer to date is the BlueGene/P, an informatics structure developed by IBM in collaboration with the US Government. It can deliver a computing power of 1 PFLOPS sustained and 3 PFLOPS peak.
The larger the botnet, the more computing power there is available to an attacker. The entire processing power can be used to perform miscellaneous tasks, not only for free, but also safe from the