This is unquestionably the most movement-specific supplementary exercise for the sumo deadlift. It helps to improve:
Muscular strength
Muscular hypertrophy
Muscular endurance
Upper back strength
Grip strength
Lat recruitment
Lockout speed/strength
Generally speaking, train it once per week during any given 3-6 week cycle. Sets and reps can range from 8-15+ depending on your individual needs and goals. Obviously low(er) rep ranges lend themselves better to strength development whereas high(er) rep ranges tend to be better for muscular hypertrophy and endurance.
The legendary strength athlete tells us why muscle growth is vital for max lifts, and why it's not the same for powerlifters as it is for bodybuilders.