By holding a ballistic plate in both the front and back (and optionally side armor) this is in effect the modern version of body armor.
Historically bulletproof vests or flak vests were used to provide protection from projectiles and shrapnel fragments – but both have been supplanted by the plate carrier.
Plate Carriers use modern ballistic plates (such as ceramics) that allow swapping of protection level to meet the mission objectives – but also to replace them after a strike.
Lastly, plate carriers are a base component of a soldier's load bearing equipment platform and as such usually have MOLLE webbing and other ways to carry additional gear.
There are advanced features of plate carriers – such as placards or front panels that allow rapid change of mission equipment.