http://bit.ly/1mS35Kk
If you cannot renew the passport from your home country for some reason (i.e. the home country's government is hostile to you), then you use the I-131 as a "travel permit" from the US in lieu of a passport.
I-131 回美證是用來取代護照，
表示得到美國政府出境的許可。
所以沒有 I-131，
可能在美國連登機都沒辦法。
http://bit.ly/1mS5B3p
U.S. law requires all customers, regardless of citizenship, age or destination, to hold a secure document to depart the United States by air (one-way or roundtrip itinerary). A secure document is a passport, U.S. permanent resident card, Refugee or Stateless travel document, Re-Entry Permit, NEXUS card, U.S Merchant Mariner Card, military ID or emergency travel document issued by an embassy or consulate.
不過這裏又寫有綠卡就可以。