Say you got a nice blue weapon and you are too low a level to equip it on your current character which happens in the crucible sometimes but your higher level character has a better weapon already why not have a place where people could spend money to buy items from other characters so people could but that rare or legendary weapon or piece of armor they've been looking for without having to hope it randomly drops from an decoherent engram or legendary this spreading around the wealth a little in the game but having a ceiling on the max price per item rarity such as 1k glimmer for rare and 5k glimmer for legendary so it also wouldn't take as long for people to gear up and they could go run with their buddies who got the lucky drops.