Military Barricades are the small structures set up on streets of towns and partially surrounding military tents. They spawn civilian grade loot as well as military grade loot depending on where they are. If a barricade is in small grouping of buildings in the middle of nowhere it normally will spawn Civilian Grade Loot. If the barricade is in a bigger town or outside of a Military Tent it will normally spawn Military Level Loot.
Military barricades can temporarily block zombies from getting to you, as it takes a while for them to jump over them. It can be a good tactic if you are overwhelmed by the undead. Another option is to engage an armed assailant because of the limited ability for your attacker to hit you, along with just enough space for you to shoot back. The only real place that a player could hit you is if they drew a bead on your head. A good way to avoid this is to move back and forth behind the barricade making it difficult for your opponent to get an accurate round off.
This poses a threat when attacking using a melee weapon, because the zombie is unreachable by any type of melee, but the zombie can somehow reach you. Make sure if you are going to engage a zombie you flank around to the other side of the barricade. Thus the zombie will be able to freely move towards you and no obstructions will stop you from taking it out. Military Zombies or any other type of zombie do not spawn with barricades, they spawn in towns and clumps of buildings which may include barricades.
Overall Barricades can play a key roll in a players survival by providing adequate, not great protection from players and zombies. They spawn different loot depending where they are located and what structures they are grouped with. The only danger the barricade poses is an obstacle for vehicles like Roadblocks. For one barricades can damage any part of a vehicle if hit, and second you can get trapped in small area allowing attackers to have more accurate shooting on you and your vehicle.
These and Military Tents are the first places most players should loot when in cities/towns.