The major accomplishments of Herodotus were to give the first true account of history and make a written record of events of the past. He is called the "Founder of History."
Before the Greeks, there was no real recorded history. There were mostly only collections of myths and stories passed down orally from one generation to another.
Herodotus changed that. When he wrote the "History of the Persian Wars," mankind had its first genuine written account of history. Herodotus was the first person to use the term "historia," which later became the word "history."
Because of his writings, mankind has a record of the past to study and learn from to guide people in the future.