Unlike Fortnite, your skins are not locked to your account or the server they are on. They can move between servers, between players and even other games. Using NFT technology, items can have their rarity, their identity, and their ownership verified at any point. This allows players to bid, buy and sell items that are actually rare.
In a closed loop system, the developers can’t verify the items total supply, much less make it something tangible enough to move between players outside of games. In these cases, the item is just code on their server. This is also the reason you can have an infinite amount of legendaries in a closed loop game. However, this makes the game more susceptible to hacks, bugs and accidental server wipes.
Years ago I got the chance to talk to the CEO of the well known mobile game Skull Girls at a Gamers Development Conference. He gave talks about moving past the darkest point of his games lifecycle: the time an employee accidentally wiped all the players progress globally. All of the players overnight found their high level characters, favorite skins. and even their favorite in-game items had vanished.
Imagine spending years and 200+ hours on a game to wake up one morning and find that all of your progress had simply disappeared. This is a perfect example of why players have sought the independence granted by an open asset system for decades.
For an open asset system to work you need to have “real” digital assets, meaning an item that can't be taken from a player or lost because of something that happens to the server. This ties back to needing an item whose ownership can be verified and validated outside of the games server.
This allows assets to survive past the game’s lifecycle and find a new home in other games.