Yes, previous years' question papers of UPSC Mathematics optional subject are helpful for understanding the difficulty level of the exam. Here's why:

1. Familiarity with exam pattern:
- By going through previous years' question papers, candidates can familiarize themselves with the pattern of the exam, including the types of questions asked, the distribution of marks, and the time duration.
- This helps candidates to develop a better understanding of the exam structure and make appropriate strategies for time management.

2. Identifying important topics:
- Analyzing previous years' question papers enables candidates to identify the important topics that are frequently asked in the exam.
- This helps candidates to prioritize their preparation and allocate more time to areas that carry more weightage in the exam.

3. Understanding the difficulty level:
- Previous years' question papers provide insights into the difficulty level of the exam.
- By solving these papers, candidates can gauge the level of complexity of the questions and assess their own preparedness.
- This helps candidates to identify their strengths and weaknesses and make necessary improvements in their study plan.

4. Practicing time management:
- Solving previous years' question papers under timed conditions helps candidates to practice time management skills.
- They can learn to allocate the right amount of time to each question, ensuring that they are able to complete the paper within the given time limit.
- This also helps in building speed and accuracy, which are crucial for scoring well in the exam.

5. Gaining confidence:
- Solving previous years' question papers gives candidates a sense of confidence as they become familiar with the exam format and the level of difficulty.
- It helps in reducing exam anxiety and boosts the confidence of the candidates, allowing them to perform better in the actual exam.

Overall, analyzing previous years' question papers of UPSC Mathematics optional subject is an essential part of the preparation strategy for the exam. It helps candidates in understanding the exam pattern, identifying important topics, assessing the difficulty level, practicing time management, and gaining confidence.