> Some monitors actually are powered from external adapter. No idea how common it is, but one is sitting on my desk.

I vaguely remember it was for regulatory reasons. I searched around and it looks like it's because with an external power supply they only need to certify the brick rather than the whole unit https://www.reddit.com/r/sysadmin/comments/24jxfj/anyone_els...

It being thinner is also a bonus.