Like Cyrus, Darius is a benevolent Persian king. When the Jews start rebuilding their temple—thanks to the prophetic urging of Haggai and Zechariah—violating Artaxerxes earlier order against rebuilding, the governor of the province asks Darius to see if there really was an order from Cyrus allowing the Jews to rebuild.
After a thorough search of the archives in different cities, Darius discovers that the order was legit. So he allows the people to continue rebuilding. He also generously grants them all the supplies and money they'll need to finish the job, along with the bulls, rams, and things they need to make sacrifices.