Handguns might appear to be weaker than PDWs at first, but they have pretty good damage at mid range.

You can use your shield to protect your head when you have a handgun equipped, which makes them particularly useful against opponents who like headshots.
They can be a life saver at close range, where just a few bullets can mean the difference between life and death.
Handguns allow you to move faster than any other gun.

Most handguns take a severe damage drop at long range, but they are extremely precise and if paired with a scope they can be very good for sniping Zombie Jammers at a distance. Try attaching a scope to a Magnum to make an ad-hoc sniper rifle.