The Bunker is the ultimate protection against all forms of attacks. It can still be destroyed by a dead on attack from large explosives (Bazooka rounds and up) it is quite the building, sticking meatshields inside of one is a good idea.
The area immediately around the bunker is a dead zone for troops because explosive weaponry will hit the immediate area around the bunker if it does not hit dead on (For example, an AT gun round).
The Bunker offers the most tactical Buffs of all available structures. It is one of the most common structures in the game as it is often used in nearly every strategy for it's ability to easily withstand grenade and mortar fire.
One of the most popular tactics, for example, uses a gunner for the bonus to firing rate and a french resistance soldier in front to soak up any incoming bullets.