Answer:
His account and specifics, aloing with his qualifications of a successful lawyer and politions making him a reliable soucre makes his account the most important. No other account of pompeii comes close to his, and they also do not have his qualifications and achievements to make their account legitimate. Furthermore, his carrear proves this isn't some roman tale, rather a real event that occured.
Answer:
Pretty simple.
Explanation:
His source was the only reliable source that was qualified for reliability. While others wrote things like "Zeus brought his fury upon us" Pliny wrote something like the mountains erupted in hot red water as black mist flooded the land. Just a perspective.