In the early days of computing many of the viruses and malware were written by “script kiddies” usually smart young kids who needed a hobby, they created viruses that would do silly things like opening and closing the cd trays it was a form of vandalism. If there was a virus on your computer back then you would know it.
These days viruses, spyware, and malware are often associated with criminal gangs and are all about the making money. One way they do this is controlling thousands of computers and creating a single “Botnet” which can be used to extort money from website owners using “denial of service attack”. A DOS attack basically makes the website nonfunctional or very slow due to the amount of useless traffic that is sent by the Botnet of zombie computers. Here’s an example how this technique can be used, lets say you owned a online store and your busiest day is Christmas Eve and the bad guys know this so they contact you and say “If you don’t give use x amount of dollars by tomorrow your site will have a very unfortunate accident on your busiest shopping day of the year”.
Much of the spam that we receive in our email inboxes come from zombie machines with the owner totally unaware that his computer is being controlled for evil deeds.
Viruses can also be used to track personal information such as credit card numbers, banking information and other personal information that can be used for very bad things.