https://www.cnet.com/news/marriott-breach-what-to-do-when-hackers-steal-your-passport-number/

The bottom line is that if your passport number is stolen, it's one more piece of stolen information about you that criminals can buy on the internet's black markets. That can help criminals steal your identity or commit other types of fraud. After another year of data breaches feeding more of our information into criminal forums, stolen passport numbers are just the cherry on top of 2018's data breach sundae. 

"Knowing this passport number is tied to an individual is valuable," said Brian Stack, vice president of dark web intelligence at Experian.