Delf explanatory.
Dirt system :
looks and function wise, meaning dirt accumulates in and or on, chamber, mags and barrel, etc perhaps lessening the functionality of the weapon or tool whatever it may be, if dropped on muddy ground mud could stick to the weapon and be wiped off by hand and still leave a small mark of mud could be washed of if submerged or comes in contact with water, hitting gun off of walls could leave scratches or small dents in metal on the weapon, making dropping a weapon accidentally feel more punished ensuring players remember to keep their guns safely held to stay in good condition.
Dirt system/parts editor (possibly) pt2 :
the feature to care and look after weapons, such as an in detail weapon parts changer, or the ability to lock weapons in a vice and clean them (such as into the radius. (if i remember correct)
Dirt system multiplayer deathmatch mode (if it ever comes) :
as you play more matches, and play for longer you weapon accumulates dust and dirt of sorts alongside scratches in metal and finishes, ties into the ability to care for weapons and possibly customise weapons visually with paints and so on.
Degradation :
after extended heavy use, such as harder force needed to rack the weapon or mis-firing sometimes or even weapon attachment failure such as flashlight and silencer failure.
Water effects :
system of submerged weapons not operating correctly such as less bullet distance/damage or maybe even complete failure of the weapon.
Why : with the dirt and other form of weapon aging/condition it is mainly for people who strive for effect of realism although a small detail plays crucial part to making the world feel more real and immersive, also a weapon editor/painter to allow for expression of creativity, with obvious moderation systems in place to report people for offensive art work and so on.