I think it's due to dubious "security" implementations, including at least one that was (?) present in a rather notorious PHP message board software package [1]

I think it's due to dubious "security" implementations, including at least one that was (?) present in a rather notorious PHP message board software package [1]. I'd be surprised if this survived all these years later, but then I've also seen some custom session handling code in my travels that did something similar even as recently as 2-3 years ago. Some image boards do it to prevent casual hot-linking, as my sibling poster notes. [1] https://tracker.phpbb.com/browse/PHPBB3-8396 https://tracker.phpbb.com/browse/PHPBB3-8396