i've been in a fill a box swap before and i find that they do it b/c not everyone is getting rid of the same things that the person giving to them wants. (did that make sense? it did to me!) lol
so lets say person A wants polymer clay stuff and is getting rid of scrapbooking stuff
person B wants crochet stuff and is getting rid of polymer clay stuff
person C wants scrapbooking stuff and is getting rid of crochet stuff
so person A gives to person C and person C gives to B and person B gives to A
thats why the fill-a-box swaps are round robin and not back and forth like normal.