Handguns are useful secondary weapons since they can be switched faster than any other weapon in the game. They are also very accurate at close to medium ranges with minimal recoil.
Handguns are ideal for poor close-range primary weapons like Sniper Rifles and Light Machine Guns. They are also useful when your primary weapon runs out of ammo mid-fight and you don't have time to reload.
M9A1
MP-443 GRACH
P226
.44 MAGNUM
PDW
Source: http://www.codghosts.net/