The PDW (Personal Defense Weapon) was designed to fill a niche in the military. Some units that have substantial numbers of troops who are trained in specialties or serve in other capacities where they do not need an assault rifle or carbine, but some may find it desirable to have a weapon that’s more substantial than a pistol. For example, troops operating communications, targeting, surveillance, or others behind the lines might still be hit by enemy special forces. The PDW gives them more firepower and range than the pistol, and it is easily carried.
The value of the PDW is open to discussion, though I like this type of weapon because it lends itself well to use by close protection teams who generally want select fire weapons that are highly concealable. PDW-type weapons have applications in law enforcement as well, especially among units that want a lot of firepower, but want it concealed until the arrest is about to take place.