The motivation behind hackers has evolved noticeably over the last couple of years. Developing harmful viruses is less about "bragging rights" or satisfying the creator's ego and is becoming more and more about generating profit or commercial return.
The destruction of data on your computer or corruption of programs you use is a common side effect and what people have traditionally associated with a computer virus. The reformatting of your computer "c: drive", especially at work, and the loss of valuable data used to be an incredibly painful experience.
The widespread deployment of data back up solutions within companies to comply with legislation and other factors means less and less valuable data is now stored on your computer's local hard drive. More importantly for the virus writer this attack does not generate much tangible profit so there is not much motivation to develop more sophisticated programs to counter improved anti virus applications and corporate network security.
However, there is profit for the virus writer in turning your computer into a spam distribution machine. "Spam" is email sent without the permission of the person receiving the message. Hackers gain control of your computer through a Trojan Horse which gives them the same access rights as the user. Once your computer is controlled by the hacker it becomes known as a "Zombie." A group of zombie machines is known as a "botnet."
By controlling a botnet a hacker can generate profit in a number of ways. The botnet can be used to exhort a ransom from a company by threatening launch a damaging "Distributed Denial of Service" (DDoS) attack against its web site. The botnet can also be hired out to other hackers.