This is customization that goes beyond pallet swapping (where the same assets are used for different items that differentiate themselves through different colors) which makes you much more attached to the gun than a bunch of polygons should. For example, a gun with a plus melee attack might have a blade on it. The guns (as well as other items) in Borderlands 2 have distinct visual features depending on their attributes.
While many loot based games are based on this mechanic, the overall appeal of new loot is boosted by the randomized look and features of each weapon and mod. Whether it's just ammo and health or a new mod, it makes killing enemies and finding chests much more satisfying than other loot based games through giving the player the potential for a good drop, which is always on the horizon. Tons of heavily customized and unique loot Borderlands 2 makes it feel like Christmas every time you kill an enemy or open chests.
