Why don't the choppers land when letting off troops?
A couple of good reasons. Firstly, because it’s cool.
In Vietnam, the Viet Cong learned to pre-fire LZs and destroy helicopters when unloading. So, the helicopter lands and the enemy pre-fires in the LZ, destroying the helicopter, killing everyone inside and the cargo inside the helicopter.
This made it harder to get supplies and troops to isolated bases in Vietnam. Cutting off the supplies made the enemy weaker and easier to attack.
So, to stop this, the U.S army and Marines would not land. Instead, they would unload their cargo while hovering so they could fly away faster. You know, just in case the Viet Cong decided to attack.
To further confuse the enemy, the U.S troops would deploy 2 different smoke grenades and only told the helicopter pilots what the correct color was. So, the Viet Cong would see 2 different smoke clouds and have to guess where to fire.
In the field, land mines also posed a threat. So the helicopter would hover to avoid being blown to pieces on landing. A land mine could kill, maim or wound nearly everyone inside. That and a downed helicopter would really hurt the mission too.
Lastly, because it’s cool. Did I mention this already? It has a certain energy to it.