Why did they need nurses in the Vietnam war?

All allied female pesonnel, regardless of their status (job) were assigned to large permenant military bases such as Quang Tri Base Camp (I Corps, 5th Mechanized Inf. Division), Bien Hoa (III Corps, Reception Center), Tay Ninh (III Corps, 25th Inf. Division), Plieku (II Corps, 4th Inf. Division), Bien Phuc (I Corps, 101st Airborne (Airmobile) Division), Chu Lai (I Corps, Americal Division), etc.

Men wounded in the field were taken care of by medics on the battlefield; from there they were walked, driven, or flown to a positioned larger force (Company/Troop, Battalion, Brigade, etc.) for further treatment. From there they might be flown or ground transported to a nearby firebase, from there or skipping the firebase (they were for firing cannons not medical treatment), straight on to the BIG BASES where all the females nurses were located.

Treatment at the BIG bases was the latest in military medical technology; extremely seriously wounded men (death imminent) had to be airlifted to "the world" ASAP.