As such, they must be garrisoned to provide an effective defence, yet – as many enemy commanders have learned to their cost – a single fire team protected behind a bunker’s walls can reap a disproportionately heavy toll on approaching forces. Many Imperial Bunkers are built as forward observation posts and are often equipped with extensive vox units and comms arrays. Typically constructed on sites with commanding views, the better to monitor the movement of enemy units, these bunkers are especially valued by frontline Imperial Guard officers who prefer to issue orders, direct artillery strikes and coordinate troop deployments from the relative safety of an armoured fortification.
Keep in mind, though, that while a Planetary Defense Force can't really man that many defensive fortifications, the Imperial Guard coming to relieve them probably has enough men to literally cover the entire planet.
It is also cheerfully decorated with the (mostly) helmeted skulls of dead Imperial Guardsmen. To provide a morale-boosting example to the new occupants.