Rashi (26-27): Ezra wrote this in his Sefer when he thanked Hashem. Until now was the letter that Daryavesh gave to Ezra to show to the Kohanim, Leviyim and Gizbarim, so strengthen them in Mitzvos, Avodas ha'Mikdash and guarding Torah properly.

Malbim: Ezra said it, both for making the king volunteer for Beis Hashem, and for what was done for him, to give to him power to strengthen the creed and set up judges.