The main reason why we identify the areas which are prone to earthquakes is two-fold:
First, we have to determine their locations to ensure the safety of the people who reside in there. In the event that major tectonic plates or faults are located in your region, the city (or municipal) engineer will need to formulate stricter rules in maintaining building strength. This is to ensure that when the earthquake comes, the casualties will be minimized, if not completely avoided.
Second, for research. Geologist, people who study the earth, are keen in determining areas prone to earthquakes as they would like to identify what geologic processes occur in this area.