I always keep my gun boxes. They sit neatly in the attic and eat nothing. Yes...they add value if or when I trade or sell the firearm that came in them.
I save anything and everyrhing that comes with a firearm purchase. I bought a used colt anaconda in 1994. It came with the blue plastic case, but not the cardboard box slip on. That slip on sells for $100 in some instances. Keep it all. U never know.
I have a friend who sold a smith and Wesson model 36 red cardboard box on ebay for 350.00!, he rand into a friend who was selling off a bunch of his fathers guns (he wasn't interested in guns) and my friend ended up getting 15 guns for 1500.00, and he told my frien he could have everything that's left, and in it was the box, and now he into 15 long guns for 1150.00!, and that's not including the other misc stuff he's sold
in the 15 long guns were 2 mosins (1 was Norwegian I think, and kinda rare in the us)
I have 2 guns that I picked up used, and if I could find the original boxes online somewhere i'd pay a hundred easily for them, cause the value on my colt detective special would jump to about 900-1000, and my bda would jump to about 700!