Let's say you're always logged in to the forum, and that would probably mean you're logged in to all the other platforms and services you use - in addition to what @ETFbitcoin (malware that steals browser cookies) mentioned as a possibility, the potential danger is that someone does come into possession of your device after all. We live in a world where it is not strange for thieves to break into our house/apartment and steal everything they can get their hands on.
In addition to someone stealing your computer, they can also steal your digital identity if you are always logged in.