Past exam papers technique can really cut the time you need to review your notes and entire textbooks because what you have is the set of questions that will likely to be included in your upcoming exams. Moreover, past exam papers give a feel/trend of exams.
If you are preparing for entrance exams like IIT, AEEE, IES, IAS, GATE, AMIE, MBA etc, then you can either purchase solved/unsolved papers from book shops or you can download these papers from websites of well known coaching institutes. However one thing you should keep in mind that generally there are errors in the solutions in solved papers, as most of these are published by petty publishers. Hence be careful regarding solutions of prblems. It would be safe to purchase unsolved paper rather than buying solved papers. You should try to get the solutions from good books or from internet. In this way your solution will not only be accurate but also be different from others.
Finally, while using past exam papers - do not speculate. You are not playing in stock market or in casino. You are studying seriously. Never guess which question will be asked and which is not. Paper setters are too tricky now a days. They can repeat the questions.
Remember that it’s highly unlikely the exact same question (same words) will come up in the exam so don’t waste time learning your answers off by heart: instead focus on identifying and understanding the key models and theories so that you can use them flexibly in an exam depending on the exact wording of the question.