One it will appraise you about the portions of the syllabus from where the questions are regularly being asked and second you may also understand the nature of the question being asked. This two factors will constantly guide you throughout your preparation of the Mains exam. So go through the question papers and identify the pattern and select the topics which are asked very frequently. Identify those topics which are particularly being asked for conventional essay type questions that require analatical abitlites and those questions which requires facts and figures. Once you have clearly demarcated the topics, the rest of the preparation will be an easy task.