system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why computer viruses exist, who profits from them, and how.
gpt
Viruses are written by different people for different reasons.  Some will write it just because they can and they get a thrill out of causing damage.  

Others will write a virus so that it can turn your machine into a 'zombie' which helps with sending out spam emails or clicking links - it is a cheap way of getting extra work machines to do your dirty work without having to pay for it.  Any money you make from the spam or link clicking is then yours. 

A few may even be paid to do it, engaging in sabotage against competitors which is a form of financial gain (by causing monetary loss to their rivals).

Also keep in mind that some viruses also gather personal information from you which can be used to gain access to your bank accounts or credit card accounts.

Another one you hear sometimes is small anti-virus companies that create a virus that infects your system, which can only be removed using their product, which you must of course pay for.