1. Efficiency . It's not the most important benefit, but it's nice to be able to get more done in less time, particularly when you have a busy schedule. Supersets allow you to get through your workout very quickly without skipping exercises.

2. Burn calories during and after your workout. When doing supersets, you’re essentially doing double the work in the same amount of time that it would take you to do a straight set but you’re working through the usual rest period. You're working on strength as well as cardio. Which triggers short and longterm burning of calories. With supersets you rest only after the superset is completed (not in between 2 straight sets). 

3. Increase muscle mass. Increasing muscle mass is what will help your body to burn more fat and get lean. The bigger the muscle the more energy (calories) you need to uphold the mass. If you eat enough protein and you're in a calorie deficit your body will target the fat cells as an energy source and you will lose fat.

4. Cardio. You're working on your cardio as well as you strength, I often use it as a cardio replacement.