Not every keylogger is a virus. There are some company which make keyloggers to monitor their employee behavior while operating their system. Also, it also has been used to study human-computer relationship in some organization.
My personal experience in using keyloggers is when I try to find out my siblings ID and password. First I installed a keylogger in her laptop and wait a couple days to find out the password. Although it mostly contains a lot of junk, after a thourough searching I finally found the session which she is logging to an account and try to log in as her. Funnily the software I used only works in internet explorer in the Mozilla firefox the key got scrambled and created an unreadable keystroke.