Using Unreal Engine 4 Hand and Foot IK

There are two main purposes for exporting FBX with Unreal Engine 4 Hand and Foot IK:

• Supports Epic IK-bone standard with additional Gun Bones for weapon-holding animations, and Root IK bones for locomotion control.
  
• Enhanced content compatibility between the Character Creator and Unreal Asset Stores. CC characters can use UE4 motions, and UE4 characters can use iClone Motions.

You can activate this feature in the FBX Advanced Settings panel.

Please refer to the tutorial video below:

The benefit for using the UE4 Hand and Foot IK can be seen clearly as the example:
The blue man (with Hand IK) holds the gun at the right hand positions unlike the orange man to the left (without Hand IK).