Using a pencil or eraser for rough work in the SBI PO Prelims Exam is not allowed. The examination guidelines clearly state that only a ballpoint pen (either blue or black ink) should be used to fill in the answer sheet and for any rough work.

Importance of Guidelines:
The guidelines provided by the conducting authority are meant to ensure fairness and consistency throughout the examination process. They help maintain a standardized approach and prevent any unfair advantage that may arise from using different writing instruments.

Specific Instructions:
The instructions provided on the examination question paper and answer sheet clearly state that rough work should be done only in the space provided in the question paper or on the backside of the admit card. Additionally, candidates are instructed not to use any additional sheets for rough work.

Reasons for Not Allowing Pencil or Eraser:
1. Clarity and Uniformity: The use of a ballpoint pen ensures that the answers are clear and legible. Pencil marks may fade or smudge over time, leading to difficulties in evaluating the answer sheet. Moreover, different types of pencils can produce varying levels of darkness, making it difficult to maintain uniformity in the evaluation process.

2. Preventing Cheating: The use of a pencil can potentially facilitate cheating as it allows for easy erasure and modification of answers. To maintain the integrity of the exam, it is essential to minimize opportunities for malpractice.

3. Efficient Evaluation: Evaluators can quickly and accurately assess answers written in pen, as they do not have to worry about any smudging or fading. This helps in expediting the result declaration process.

4. Standardized Approach: The SBI PO Prelims Exam is conducted on a large scale, with thousands of candidates appearing for it. To ensure fairness and uniformity, it is crucial to follow the prescribed guidelines, including the use of a ballpoint pen for all writing purposes.

Conclusion:
It is essential to adhere to the guidelines provided by the conducting authority and use a ballpoint pen for all writing purposes during the SBI PO Prelims Exam. Using a pencil or eraser for rough work is not allowed due to reasons such as maintaining clarity, preventing cheating, enabling efficient evaluation, and following a standardized approach.