AFCI breakers are designed to detect low-level arcing faults that can persist for long periods of time with current levels below the threshold of normal circuit breakers. These are the types of faults that cause many fires, particularly in areas such as a bedroom with combustible blankets, bedspreads, curtains, etc.
As far as coordination, these breakers are usually going to be the last breaker in a circuit. Since they will be much more sensitive than the regular upstream breakers, there should be no coordination problem. If one of these devices is used upstream of a regular breaker, coordination is most likely not possible.