There are many different goals. Here are some examples:

• Warfare -- Not long ago a computer virus was found that did nothing except spread itself quietly, until it got into an Iranian uranium center's control computers, at which point it destroyed a bunch of nuclear enrichment equipment. It was incredibly complicated and very likely the work of the US and Israeli governments.
• Ransomware -- some viruses encrypt a bunch of stuff on a machine and prompt the user to send money somewhere in order to receive an unlock code.
• Botnets -- It can be useful to have an army of a million computers for many reasons, from performing calculations to sending spam to attacking other machines and so forth. Some viruses spread and do nothing but are designed to be issued further orders from some control point, effectively making them zombie slaves to the hacker for use at some later time. Some of these fleets are rented out as a product ("I will sell you six hours of control of 100,000 unwitting computers for you to send out spam").
• Mayhem -- Some people just want to watch the world burn. They create viruses designed to cause havok, like the ones that quietly spread until such and such date, at which point they crash as many computers as possible simultaneously.
• Theft -- some hackers create viruses that spread out and call home, not to be used as a botnet, but so that hopefully some of them end up somewhere profitable, like with access to people's credit card info, or better yet a business's list of customer credit cards. Some malware watches your web browser, waits for you to go to your bank site, then remembers the password you type in and calls home to report it.
• Research -- many universities provide proofs-of-concept demonstrating a flaw with some security practice or other. Occasionally these get let into the wild accidentally, or more frequently someone malicious reads the research paper and implements the described exploit for one of the other reasons above.

Wow, thank you for the informative reply.