1. When you boil your whole wort, it's all sanitized. As oppose to adding cold water. But for that you only need to boil for 15 min
2. Hop utilization is lower when the wort is more concentrated, eg. when you boil enough extract for 5 gallons of beer with only 2 gallons of water. Boiling the whole amount will help you utilize the hops better.
Plus... A big honking pot full of boiling beer is just cool...