You don’t “have to”. They’re requested when travelling to the US - but from my experience it’s not enforced (at least not consistently, I’ve never been asked why it was blank on my forms)
I thought so too, and said so at a meeting at work when some colleagues were unwilling to travel to the US.
It turns out close contacts of some of my colleagues have been asked to show their Facebook accounts at the US border. There was a recent rule change about it.
