The Humvee is an upgraded jeep, having the ability to load 6 foot units instead of 4. Along with the extra space, they allow protection. Like the Jeep the Humvee takes up one soldier spot.
A humvee and a jeep can together hold 10 soldiers instead of having them all in a soldier transport without the ability to shoot. If you max out your solider count, 2 Humvees with 6 heavy soldiers each makes for a very powerful attack force. Another effective strategy is to use snipers with 2 humvees.