Featherfall is a survival gear perk that cuts the damage you take from falls. It helps when you jump off rooftops, cliffs and watchtowers to escape.
You take less damage when you drop from a height. Short falls that would normally hurt become safe.
A comfort perk more than a combat one. If you already use the wingsuit and parachute well, a speed or stealth boot perk helps more.
High Ground, also on Altaïr's gear, rewards you for attacking from above.