Put another way, it is one step beyond a green card holder. Citizenship will give you the maximum rights available in the U.S. Citizens may also find it advantageous to use a U.S. passport when traveling. The new immigration law brought new advantages to citizenship. The new law severely limits public benefits for legal permanent residents. With full citizenship these limits disappear.
Citizens have much greater ability to sponsor relatives for U.S. immigration.
Many government contracts and jobs require U.S. citizenship. One cannot get a voting card without becoming a citizen. The new law severely limits public benefits for legal permanent residents. If you become naturalized you do not have to worry about replacing your Green Card with newer versions. For example, the INS announced the expiration of the old Green Card forms I-551.