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.”
R’ Yehonason Eibshutz explains that what the Jews began doing was a continuation of what they started doing at Sinai. For this reason, although there is a prohibition (Devarim 13:1) to add mitzvos to the Torah, Mordechai had the authority to add the mitzvos of Purim in this case. The nation achieved atonement because they listened to Mordechai, and the sincerity of their re-acceptance of Torah allowed for the Torah to be new.
The Sfas Emes and Maamar Mordechai both note that in this verse, too, the Jews did before they learned, in the same fashion as their saying (Shemos 24:7) “naaseh v’nishmah” (“we will do and we will listen”) when they first accepted the Torah.
The Vilna Gaon explains that there were three things Achashverosh performed for Haman; he made him great by making him financially wealthy, he elevated his by giving him more authority, and he gave his a higher seat in that his advice had superiority over that of Achashverosh’s other advisers.
The Yalkut Shimoni (1053) and M’nos HaLevi say that these three actions indicate that Achashverosh gradually gave Haman new powers every day. The Yalkut Shimoni actually goes on to say that Haman eventually had even more power than the king, Achashverosh building for him a throne even higher than his own. Rav Avie Gold points out that the gematria of Haman (5+40+50=95) is equal to that of “hamelech,” (“the king”) (5+40+30+20=95), implying that they became equal to each other. Furthermore, the gematria of Haman’s entire given name here, Haman son of Hemdasa the Aggagite, (5+40+50+2+50+5+40+ 4+400+1+5+1+3+3+10=619) is one less than “kesser,” (“crown”) (20+400+200), implying that he was just within reach of the royalty he desired.
The Malbim points out that Haman’s gradual ascent to power is similar to what Yosef tells his brothers (Bereishis 45:8) regarding the three steps of his promotion. The Alshich explains the reason for this (in both cases, perhaps) is that king did not want his others advisers to harbor jealousy toward the new “upstart.”
Perhaps another reason for this gradual elevation is that Achashverosh, himself, was not a product of royalty, but was a self-made man, and felt that this was the proper way for somebody to advance.
The Vilna Gaon points out that this reference to the “king,” is, again, H-Shem. In His Wisdom, H-Shem elevated this wicked man in order to provide the platform for the Purim miracle.