That question came up to me, when recently working on one project I needed to restart my PC several times. And therefore, while I used 3-5 web apps I needed to log back in to each one of them again, after each restart. And I started wondering if privacy-wise that auto-clear feature is worth it or not? Has anyone maybe tested that?
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”.