Short answer: You should use a password manager because good passwords are hard to remember. Long answer: See below.
To log on to a website on the internet, you normally have to provide a username and a password. A good password is a long one because the more characters a password has, the longer it takes for a hacker’s computer to guess it. But it’s also generally hard to remember long passwords, and many people tend to use the same password – and often username – on all the websites they log in to.
When you use the same credentials everywhere, there’s a higher chance a hacker can figure out your username and password.
Actually, it’s very likely that it has already happened.