During the War against the GLA, many poor civilians throughout Central Asia and the Middle East were experiencing troubled times - famine, sickness and the war itself were at the center of them. The United Nations and the Red Cross, out of goodwill, proposed to relieve them of their burdens.
Stored in large crates bearing the initials of the UN, these crates contained supplies and other necessities - drinking water, food, medicine etc. - and distributed amongst the downtrodden people of these areas.
However, these supplies were occasionally "borrowed" by the three warring armies so as to avoid being short of funds and war material in their struggle against each other. (Chief among them the GLA, who frequently raided UN humanitarian relief workers and their convoys, along with pillaging the homes of dozens of civilians whom they perceived as stealing "their" supplies).
Throughout the war, United States commanders with the appropriate clearances were able to request more of these supply crates through Supply Drop Zones to assist their war efforts.
Much like the previous iterations of crates in the Command and Conquer series, these UN Crates grant the player a boon upon successfully acquiring them. These crates, however, are the only ones in the franchise that provide no other bonuses except extra funds, usually in the range of $1,000-$2,500. The USA faction is capable of obtaining additional UN Crates through a Supply Drop Zone.