Edit: I thought FWH just added some more flavor.....
Flavor is the reason I use it, too, but it does add some bitterness. According to analyses I've had done, about 10% more bitterness on average than a 60 min. addition. But neither I, nor a lot of the testers in the experiment I did, perceive FWH as more bitter. In fact, a lot (most?) tasters found the all FWH beer to be less bitter than the same beer with only a 60. min. addition of the same amount of the same hops.