If you're sharing the computer with others and you don't delete cookies after having logged in to whatever website/service that you were using, the browser could automatically log another person in with your credentials. This is because cookies can store "sessions". While I'm not a fan of Linus himself - and I do implore you to make up your own mind on that -, one time, at bandcamp, they almost lost their whole YouTube channel because their session tokens were grabbed, and they made a great breakdown of the incident, which can be educational: https://www.youtube.com/watch?v=yGXaAWbzl5A

I make my browsers either not save cookies at all or delete them on exit by default and manually tweak site settings for specific sites if and when I want to be able to log in "automatically".