"With this money be sure to buy bulls, rams and male lambs, together with their grain offerings and drink offerings, and sacrifice them on the altar of the temple of your God in Jerusalem." NIV translation
He told Ezra to use his money to buy the things required for the offerings in the temple. He hoped to gain the favor of God because of his treatment of God's people and he was right in principle because God does bless those who bless His people.
"You and your brother Jews may then do whatever seems best with the rest of the silver and gold, in accordance with the will of your God. Deliver to the God of Jerusalem all the articles entrusted to you for worship in the temple of your God. And anything else needed for the temple of your God that you may have occasion to supply, you may provide from the royal treasury." NIV translation
The king was very generous and pledged all the royal funds necessary to purchase things used for worship in the temple. He repeatedly identified God as "your God" and so we know that this was not done by the king as an act of worship.
"Now I, King Artaxerxes, order all the treasurers of Trans-Euphrates to provide with diligence whatever Ezra the priest, a teacher of the Law of the God of heaven, may ask of you - up to a hundred talents of silver, a hundred cors of wheat, a hundred baths of wine, a hundred baths of olive oil, and salt without limit. Whatever the God of heaven has prescribed, let it be done with diligence for the temple of the God of heaven. Why should there be wrath against the realm of the king and of his sons." NIV translation
The king gave orders for the local provinces to provide the things for the temple. We finally get to see the reason for this generosity and it was simply fear. Artaxerexes feared the wrath of a God that he did not even personally know. As it says in Proverbs 9:10, "The fear of the Lord is the beginning of wisdom" and so the king is making some wise decisions based on his fear. It seems that this a forgotten aspect of our coming to faith in God. We hear much today about the love of God but we do not hear much about fearing His wrath if we don't turn to Jesus. Without fear of the consequences of sin and God's coming judgment, we will not repent and accept Jesus' payment for our sins.
"You are also to know that you have no authority to impose taxes, tribute or duty on any of the priests, Levites, singers, gatekeepers, temple servants or other workers at this house of God." NIV translation
We see how afraid the king was of God's wrath in the fact that he basically allowed the temple workers independence from the other authorities.