Many people seem to leave their computers permanently logged in to online services. If your hard disk isnt fully encrypted, a determined thief might be able to access your data after stealing the device. The moral? Remember to log out. If you want to do a better job of keeping your information secure, use some kind of full disk encryption. If your computers hard disk is not encrypted, it might be easier for a thief to steal your data without your knowledge of the password.”]
Source: https://nakedsecurity.sophos.com/2013/04/22/xkcd-cartoon-log-out-security/