According to the US Department of State, second passports can be issued to people who make frequent international trips that require multiple visas.

For example, if you’re planning on traveling to a country that requires sending your passport ahead of time for a visa, then not having your passport would definitely restrict your travel during that time.

In my case, I carry a second US passport because my first passport might also contain visa or entry stamps of countries not recognized by other countries. And of course, the third reason is that if I ever lost one passport, I could still travel.

One caveat is the second passport is only good for four years instead of 10.