יא אֲשֶׁר נָתַן הַמֶּלֶךְ לַיְּהוּדִים ׀ אֲשֶׁר בְּכָל–עִיר–וָעִיר לְהִקָּהֵל וְלַעֲמֹד עַל–נַפְשָׁם לְהַשְׁמִיד וְלַהֲרֹג וּלְאַבֵּד אֶת–כָּל–חֵיל עַם וּמְדִינָה הַצָּרִים אֹתָם טַף וְנָשִׁים וּשְׁלָלָם לָבוֹז
11. “That the king gave to the Yehudim who were in each city and city to gather, and to stand on their souls, to annihilate, and to kill, and to destroy any army of the nation and state who antagonize them, infants, and women. And their property they should plunder.
According to Dena Pishra, the letters required the Jews to gather because Jewish unity is a powerful method of attaining H-Shem’s assistance.
The Sfas Emes adds that Jews have an aggregate soul, and our combined effort is especially needed when we are in danger.
Furthermore, the Ginzei HaMelech asks that if the word “unify” requires two or more people, and the word “stand” implies one [as one person stands for oneself], which is the letters’ intent? Based on Rashi’s comment on the Talmud (Shabbos 127b) that “stand” means to pray, he answers that the Talmud (Bava Kama 97a) promises a Jew that when one prays for another’s needs, one’s prayers are never rejected. The Talmud (Brachos 8a) says the same idea regarding communal prayer.