For example, the INS announced the expiration of the old Green Card forms I-551. All people in possession of the card had to apply for replacement with a secure, machine-readable Alien Registration Receipt Card. Citizens do not have to do this. Only citizens can take out U.S. passports. Entering the U.S. is easier.
Many countries waive visa requirements for U.S. passport holders.
With a U.S. passport, you are eligible for U.S. citizen services from U.S. embassies and consulates when traveling throughout the world. U.S. citizens do not have to carry proof of citizenship. On the other hand, INS feels that permanent residents must always carry their green cards. INS has detained permanent residents who forgot to carry their cards. Finally, there are intangible benefits.