I had a password that consisted only of letters, I used it everywhere. Sometimes sites would ask me to put numbers in my passwords, so I made a different one. Sometimes sites would ask me to put some symbols, so I made a different one. Sometimes sites would ask for a longer password, or I wanted a safer password, so I made two new different passwords.
Now whenever I want to login on a site I don't usually do, I type them all, in order, beggining with the least safe password I invented 12 years ago.
Imagine if the website I'm trying to log in saved these attempts? It could use them to log in my accounts on all other sites of the internet.