Designing a combat knife with holes running down its length is done mainly to make the blade lighter - after all, lesser metal means a reduction in weight. Also, there is a belief that making holes in combat knives makes them more efficient weapons. This is because the holes let the air out through wounds inflicted with the blade, thereby allowing them to be withdrawn from the body cavity much quicker. In cases where the hole is on the handle, it is merely to provide an avenue for attachment of a lanyard, which makes the knife more comfortable to carry. This lanyard also provides a greater reach during physical combat, allowing you to deal with adequate damage from a greater distance.