Basically, it’s like the item stores for CounterStrike and TeamFortress2 extended to all Steam games that will support it. Gamedevs can launch their own curated items marketplace with users-based content, sell them and share the profits with the authors (I suppose Steam takes its share too). Kind of like a potentially infinite supply of micro-DLC and micro-transactions.
So if you intend to do a “customizable” game for Steam you might want to think about the implementation of this feature for more cash-grabbing.