Swords did not evolve in a smooth transition from one form to the next, but early versions of this sword seemed merely to be an elongated version of the arming sword. The longsword, with its 36 inch double-edged blade and 10 inch hilt, weighed in at 4 to 6 lbs. While lighter versions yielded the option of using one or two hands, heavier ones necessitated two.
Knights often carried shields into battle, but as plate armor grew to cover larger areas of the body, shields grew obsolete, since basically, knights were wearing them. Hence, the shield hand was freed up to grasp the longer hilt and the knight wielded the longsword with double-fisted force.
While it can be said that all parts of any sword can be used offensively, the oversized pommel and crossguard of the longsword proved quite effective. And any advantage may lead to victory.
In tightly compressed combat however, the sword could be rendered useless as knights struggled to find room to swing this longer weapon. This was a factor in the defeat of the French knights at the battle of Agincourt. Well that, and a whole lot of mud.