Well I've never heard its use phrased quite that way before. Amprolium is a very common medication for chick starter.
The idea of medicated starter is that it will keep the amount of cocci in the chicks gut in check, but not totally eradicate it. This gives the chicks own immune system time to mature and develop immunity. A body has to be exposed to something for the immune system to kick in.
When I brooder raised chicks, I fed them medicated feed with amprolium AND got them outside as soon as possible to be exposed. The amprolium being a sort of insurance against the coccidia overwhelming the chicks system and causing death.
Now that my broodies raise the chicks I no longer feed medicated. The chicks are slowly exposed to coccidia by walking in the adults birds droppings and being outside, so it's a gradual exposure. Does basically the same thing as feeding medicated chick starter.