This is a system that has been around for quite awhile. It is almost a staple in any dungeon crawler that has been made in the last 20 years. The common system for loot in these games is to vary the quality of an item by how rare that item is. This is good design because it solves multiple problems that a designer could have when making a dungeon crawler. The first problem it solves is it gives you as the designer more control of balancing for different zones in games. If players find a specific area difficult. You can increase the drop chance of stronger weapons in that area or the one before it. If one zone of the game is less traveled. Add a rare item to that zone to increase player traffic. The biggest function this system offers is that it gives the players a scale for the items they have. This is great because if a person doesn't have the best or rarest item from the area they have just completed, it gives them reason to go back and try and find the better items.
World of Warcraft uses this systems and benefits from it greatly. In WoW players will go back and solo instances and raids that have existed for more than 10 years. They will do this week after week with the soul purpose of looting a rare item that looks different from other items.