Not everything you do in bodybuilding is directly intended to add mass. There are subsidiary goals, which are actually interdependent with adding mass. Increasing strength for example. Adding strength and adding mass are interdependent, you can't really do one without doing the other.
Another subsidiary goal is improving conditioning. Better conditioning will allow you to do your sets with shorter rest time and better form, & potentially get more and better quality work done in the same amount of time.
To me that's the biggest benefit of supersets. Since it's about conditioning, you need to choose opposing exercises that allow you to rest one muscle group while working a different one. That allows you give a good effort on each exercise but still up the overall energy demand and so work to increase conditioning. For instance today I did a superset with RDL + leg press. There's a bit of overlap there but it's basically hamstrings vs. quads. You move from one exercise to the other with no rest, performing 1 set at a time.