At one point, hacking was done for bragging rights. There is an art to the creation of software worms that can propogate far and fast, and the prestige that goes along with obtaining access to private networks is its own reward. Today, many worms are written for profit. To be infected with a worm or virus usually means that your machine has one or more bots covertly running. These are small, semi-autonomous programs that can hide themselves, stealing bandwidth and processing power to do evil things like sending spam, scanning for passwords and credit card information, attacking and taking down web sites, or spreading copies of themselves to other machines. They are the major cause of mysteriously slow computers and internet connections, with tons of things going on in the background that are invisible to the casual user.
Bots are useful when they work in teams, larger botnets can coordinate attacks and distribute resources more efficiently. And the people who write and control botnets sell, rent, trade and steal them from each other in open warfare and direct competition. I've written here before about botnets, but its worth mentioning this article: Know Your Enemy: Tracking Botnets. It was written by an international research alliance of computer science and IT professionals, whose board includes the Senior Legal Director at Yahoo. the article is a highly readable introduction to bots, the things they do, and the people who control them.
Human Botnets I
Human Botnets III
Human Botnets IV
Human Botnets V