There are several possibilities. The lower portion of the grip is thinner and therefore weaker, so it could have been done as a means of reinforcing that area of the grip. With any two-handed sword, be it a longsword, great sword, etc., the forward hand controls direction and the trailing hand is responsible for power. Consequently, the wire will provide a firmer gripping surface while the leather will allow the control hand to shift more easily to direct the swords motion. Finally, since weapons were considered a form of man jewelry it's entirely possible they just thought it looked cool, or a combination of all of the above.