In Afghanistan, compared to Iraq, the distances ground troops have to travel to stage a raid, are longer, the surrounding ground is higher, and the population is more spread out in rural areas. The result is that the enemy has an easier time spotting you coming, and a better chance of getting away. In Iraq, even though the enemy might have lookouts, the more urban landscape made it difficult to determine if the approaching American column was headed for their guys, or someone else. Thus in Iraq, most raids were conducted by troops travelling by road. In Afghanistan, it's more often necessary for the raiding troops to go in by helicopter. While vehicles will usually come in by road to return the raiders to base, this extraction method is often called off. That's because if the UAVs or helicopters spot many Taliban rushing to set up ambushes or plant mines and roadside bombs, it's often preferable to take the raiders out by air. While it's possible to attack the Taliban ambush forces from the air, there are often not enough UAVs, aircraft and helicopters available to clear the way.
This use of helicopters is the main reason NATO troops are loudly complaining to their governments about their shortage of helicopters. While the reason for more helicopters is usually given as the need to avoid roadside bombs and ambush (especially to supply convoys), this is only partially true. The use of helicopters to gain surprise, and help keep an eye on the roads, is the big thing. The troops are there to fight, and the helicopters help them do that better. Cutting casualties is important, but not as crucial as getting at, and defeating, the enemy.