A lot of people look at med ball work as a way improve/trains the anterior core for power, which it does.

A lot of people look at med ball work as a way to train a very sport-specific pattern that mimics the high speeds that are similar to those seen in sport, which it does.

A lot of people look at med ball work as a way to help to teach an athlete how to properly create forces from the ground through the legs, through the core, then finally out of the arms in a similar fashion to sport.

But maybe most important and often overlooked aspect to med ball work for a volleyball player is from an injury prevention and shoulder health standpoint, as an overhead throw/slam or shot put teaches the posterior shoulder to decelerate appropriately once the med ball is released, in a similar way that will be seen when hitting a volleyball.