As Voi is an actual city Bungie was able to use the town as a piece in their viral marketing ARG (Alternate Reality Game) IRIS in the lead up to Halo 3's launch. Near the beginning of IRIS Bungie gave the Longitude and Latitude of Voi as a hint for the ARG.
Voi is also featured as a central city of the Halo 3 campaign.
After the UNSC base Crow's Nest is assaulted by Covenant forces, and subsequently destroyed by the UNSC with a timed bomb, Master Chief and a battalion of marines travel toward Voi through Tsavo Highway. Once they reach the city, they destroy a Scarab, several Anti-Air Wraiths, and an Anti-Air Gun, enabling Lord Hood and a UNSC fleet to attack Truth's Forerunner Dreadnought. But they are too late, as the portal opens, and the Covenant ships teleport to The Ark.
Almost immediately after, a flood commandeered Covenant ship slips into view, and crash lands in the city. Master Chief and the Arbiter fight through waves of Flood, until they eventually witness the Elite ship, Shadow of Intent, drop Elite troops into the area. Master Chief soon receives word from Miranda Keyes and Sergeant Johnson that the Elites believe Cortana is on the Flood ship. Master Chief and Arbiter reach the ship, then Master Chief goes inside and retrieves what is actually only a message.
Onboard Shadow of Intent, Master Chief, Arbiter, Lord Hood, Miranda Keyes, Shipmaster, and various others listen to Cortana's message. She says that an army of Flood is headed to Earth. In addition, Cortana claims that, unbeknownst to Gravemind, on the other side of the portal lies a solution for expunging the Flood.
Rtas 'Vadum glasses the city of Voi, and the allied Covenant Separatists and UNSC travel into the portal, aside from Lord Hood, who stays behind to protect Earth.