Less harsh bitterness, you get more aroma/flavor from your hops. I've been doing it on almost all my beers and have to say I LOVE it! In my mind it helps to get the most out of my hops.
First wort hopping means that some of the hops go in the kettle and then the mash tun is drained onto them.
So the hops are soaked in hot-but-not-boiling wort for a while (15 to 60 min).
Some of the flavor that would normally boil away is retained and it gives a more mellow bittering effect.
Most calculators also put the IBUs of it 10% higher than if they were just added to the boil, but the perceived bitterness is that of a 20min add IIRC.