Observant is a loot gear perk that lets you collect ammo and Pesos from dead soldiers from much further away. You stop walking over to every body.
Ammo and Pesos from dead soldiers get picked up from a much longer range. You resupply mid-fight without leaving cover.
Yes, it is a big quality-of-life boost in long fights.
Greed and Peso Accountant increase what you collect.