The "medicated" part of the feed is a coccidiostat.
Coccidiosis is a disease birds get as a result of a protozoan infestation, very similar to malarial infiltration. Young birds are particularly susceptible.
This disease is passed from bird to bird, through droppings and infected tissue. It causes untold intestinal distress, dehydration, and if left unabated, will quickly kill a flock. It is primarliy the result of overcrowding on the same piece land, season after season. In other words, improper management and poor land utilization.
In a nutshell, where it gets hold, all the birds kept here are susceptible.
The coccidiostat breaks the life cycle of the protozoan vector and so, in theory, removes the threat. In the end, its one of those things that cant hurt, even if it isn't needed in your particular case.
Get it, if you can.