The purpose of safety modules/PLC's is all about the ability of detecting errors. These modules have redundant monitoring to detect errors such as ground faults and short circuits. There are higher costs associated with safety systems because of the redundancy and a safety PLC also reduces the functionality of the programming by not allowing some of the higher level languages like structured text. For additional information on safety modules in general just do a search like "why are safety modules used in plcs". Info for S7 specifically, search for "safety modules used in s7 plcs".