Using previous years' papers for practicing in-depth analysis and critical thinking can be a valuable strategy for preparing for exams, such as the UPSC. Here are some detailed explanations on how and why it can be beneficial:

1. Familiarizing with Exam Pattern:
- By practicing previous years' papers, one can gain a comprehensive understanding of the exam pattern, question types, and the level of difficulty.
- This familiarity allows candidates to prepare strategically, focusing on areas that are frequently tested and allocating appropriate time for each section.

2. Identifying Important Topics:
- Analyzing previous papers helps in identifying the important topics and subjects that are frequently asked in the exams.
- By recognizing the recurring themes, candidates can prioritize their study material and allocate more time to topics that carry more weightage.

3. Developing Analytical Skills:
- In-depth analysis and critical thinking are crucial skills required for answering questions effectively.
- By practicing with previous years' papers, candidates can enhance their analytical skills by understanding the nuances of different questions, identifying key concepts, and formulating well-structured answers.

4. Time Management:
- Solving previous papers provides an opportunity to practice time management skills.
- Candidates can learn to allocate the right amount of time to each question, ensuring that they complete the paper within the given time frame.

5. Gaining Confidence:
- Regularly practicing with previous years' papers builds confidence and reduces exam anxiety.
- Candidates become familiar with the question format and develop a better understanding of their strengths and weaknesses, which helps them approach the actual exam with a positive mindset.

It is important to note that while practicing with previous papers is beneficial, it should not be the sole method of preparation. Candidates should also refer to textbooks, study materials, and other relevant resources to supplement their understanding of the topics.

In conclusion, using previous years' papers to practice in-depth analysis and critical thinking can greatly enhance exam preparation. It helps in familiarizing with the exam pattern, identifying important topics, developing analytical skills, managing time effectively, and gaining confidence. However, it should be combined with other study resources to ensure comprehensive preparation.