I'm sure it was so the pilot could see the ground when he was on his landing approach. The XF5U was supposed to land at a very high angle of attack, probably eliminating the pilot's visibility of the deck. A transparent nose cone eliminates that problem. If you look at the V-173 (the proof of concept aircraft which led to the XF5U), it has even more glass on bottom of its nose to provide good downward visibility for the pilot.