Ask HN: What malicious things can someone do with a copy of your passport?

In a currently popular HN article about someone getting all of their stuff stolen[1], they mention that a person likely got hold of copies of their passport. I have also had concerns about someone potentially getting their hands on one of the many copies of my passport I have floating around. Should I be concerned though? What malicious things could a person potentially do with the information on a passport (US or otherwise)?

[1] http://levels.io/100-to-0-things/