Not exactly right. There is a low level of amprolium in medicated chick feed. Amprolium is a coccidiostat, which means it will prevent the protozoal organism, coccidia, from being able to reproduce. The body's own immune system in the gut must then attack the organisms themselves, which results in an increased amount of antibodies circulating in the chicks and thus better protection against coccidia once the medication is stopped. And coccidial organisms are commonly found in the dirt, healthy flocks, etc.
The level of amprolium in feed is not high enough to treat an outbreak of coccidiosis. If there are enough organisms present, it will not able to prevent all of them from reproducing. In this way, it is a preventive measure only. If you have an outbreak, you will have to switch to something like Corid to treat properly.
Using a medicated chick feed is largely an individual preference. Some people want only the chicks with strong immune systems to survive so do not use it, some might have a low environmental load of the parasite so don't need it as much, some keep the chicks in a disinfected indoor brooder until they're 6 weeks old so they wouldn't be as susceptible to an overwhelming infection, some use different immune-boosting methods instead...etc.. Some feel that it gives the chicks support through their most vulnerable time of life, some have high loads of coccidia in their flock, some brood outdoors, etc. and choose to use it.