Past papers are an important tool for revision, allowing students to check how their revision is going and assess areas for improvement. Just doing the questions, perhaps under timed conditions and consulting the mark scheme is useful.
past exam questions give you the idea of number of question and the available choices. The time for solving the exam paper can be efficiently managed if you have the idea about the pattern of the exam paper. It is therefore necessary for students to give some time to go through the past paper while revising a subject.