There is only one purpose for using a sumo stance. Move as much weight as possible with as little muscular work as possible. It’s not a “bad” exercise per se, but completely inefficient if your goal is building muscle, losing body fat, or improving muscular strength.
This is why it is sometimes used in the sport of powerlifting, where moving load through an arbitrary range of motion is the only goal. However, in that case the only reason to do a sumo stance deadlift is to practice the skill of the movement. If you want to get stronger at the sumo deadlift, the most efficient way is not doing the sumo deadlift. It would be utilizing other exercises, like a conventional deadlift, leg press, RDL, even leg extension to strengthen the muscle tissue that contributes to the movement itself.
To be clear, the sumo stance is not the same thing as a wide stance deadlift.
When it comes to aesthetic goals, such as muscle gain or fat loss, this exercise is a complete waste of time. The neurological demand far outweighs the muscular work being done. More muscular work is the goal of hypertrophy or fat loss training. The purpose of the training is more tension or metabolic stress within the target muscle tissue (depending on the goal of the workout). Don’t choose an exercise that minimizes muscular tension and maximizes load on your joints.