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.
A competitive powerlifter recently contacted me to correct his woefully pathetic bench press. After a few tests and some obvious visual clues, I realized that his triceps were holding him back from pressing up big numbers.
Start your biceps work with hammer curls, then a preacher curl variation like the one here. For safety, finish with heavy barbell curls when you're biceps are fully pumped.