How many times do you have such situations when you were kicked from admin panel because of session timeout? You could work on a content for a product or a static page. Perhaps, it took long or you took a coffee break. But after you clicked save button, you were logged out. As result, you lost the work you had done and you had to log in again and did the same from scratch. By default PHP session is expired after 24 minutes of inactivity and it doesn't matter what browser cookie lifetime you configured for admin.
If you want to stay logged in all the time until you log out yourself, we have a very simple but smart solution for you. It's the module Keep Admin Authorized. Now you don't have to worry about being logged out automatically. It works in a way that every minute Ajax request is sent to a server to keep your PHP session and browser cookies active.