In Torah Nation (pg. 40-1), R’ Avigdor Miller explains that the verse uses the word, tokef (authority”), because Esther used her authority as queen to make sure the Jews knew the seriousness of their accepting her words.
Rashi seems to translate the word as “power,” and explains that the verse is hinting to the power of the Purim miracle’s effect on the principle players of the story, Achashverosh, Mordechai, Haman, and Esther.
The Ben Ish Chai suggests that the events in which the different characters rose to power are the reasons for the different opinions in the Talmud’s (Megilla 19a) theoretical discussion regarding the point in Megillas Esther from which one is required to read during the public reading on Purim.
M’nos HaLevi writes that Esther needed to reinforce the establishment of Purim with her authority because it may become difficult in future generations to keep the holiday, but it must nevertheless be celebrated.
The Midrash (Rus Rabba 2:4) notes that Jews outside of Shushan reacted negatively to the first document, so this second letter needed to be stamped with authority.
Malbim, focusing on the fact that the verse says, “kol tofek,” or “all the authority,” explains that the letter needed two different kinds of authority; the throne’s to be published, and Mordechai’s to make it part of the TaNaCh canon.
Rav Schwab adds that Esther is called a queen here to give legitimacy to Daryavesh, her descendant.
In response to the rabbis’ question in the Talmud (Megilla 7a) about why Megillas Esther needs to be read like a Torah scroll, Esther convinces them that it is much like the Torah in that both are concerned with the war against Amalek. This furthers her argument that Megillas Esther belongs in TaNaCh, since it is written with ruach hakodesh.
R’ Elisha Gallico writes that Esther wanted Megillas Esther in TaNaCh because she was married to a gentile, and wanted future generations to know what led to such an unfortunate situation.
In Keemu v’Keeblu, Rav Brevda likewise writes that this was the reason it was in Persian’s royal chronicles. Ancient chronicles were often not objective, so the very presence of this story in the royal chronicle was proof that the king approves. Then, rightfully, if we were to be derided for celebrating this holiday, we could respond that “we Jews celebrate because the king celebrates.”