There are three main reason why Malware programs are created:
Profit is the main reason that most malware programs are created. Malware can skew search engine results and display links to pages that are paid advertisements operated by devious entities. They can drive traffic to pages that offer products ranging from virus fixes to cheap medications and/or collect information about you that is then sold to marketers.
Profits can also be generated in a more nefarious fashion: by searching your cache for Email addresses that are then collected and sold to spammers. Malware can also collect stored information that is transmitted and sold to organizations that engage in identity theft and/or credit card fraud.
DDOS attacks from botnets are often used to take down large, e-commerce Web sites when the victims refuse to pay a ransom.
As Pierre Choderlos de Laclos once wrote: “Revenge is a dish best served cold.” Serving it up cold is just what computers are good at doing. Viruses can look for financial data stored on hard drives and create minor anomalies such as adding or subtracting numbers at will and sending your accounting department into a tailspin. Imagine being off by a million dollars in your corporate audit or sending an important client an inaccurate statement! Viruses can also attack systems such as Web Servers using DDOS attacks a bring Internet based companies to their knees, causing great financial losses.
Hackers have monikers (such a Neo in The Matrix) that they use to brag anonymously. Breaking into an important database or causing great havoc can bring a lot of fame and recognition within hacker coterie.