I will say that even when going into combat today, we wear multiple layers of protection.
Helmet + another hat underneath (though, admittedly, the underhat is more for comfort than anything else)
Shrapnel resistant goggles
Undershirt
Uniform shirt
Flak vest, which generally includes removable shoulder protection, and dangling protection for the nether regions
Elbow pads
Gloves
Underpants (for cleanliness!!)
Uniform pants
Knee/shin protection depending on the mission
And, obviously, boots (again, various kinds depending on the mission)
So I think there is obvious realism bearing on the multiple armor slots and once you get into the customization/fantasy/+1 mindset, rings, amulets, earrings also become a factor.
Of course, I am also a collector type player... inventory management is one of my favorite things about games besides the story for me.