ח וְאֶת–פַּתְשֶׁגֶן כְּתָב–הַדָּת אֲשֶׁר–נִתַּן בְּשׁוּשָׁן לְהַשְׁמִידָם נָתַן לוֹ לְהַרְאוֹת אֶת–אֶסְתֵּר וּלְהַגִּיד לָהּ וּלְצַוּוֹת עָלֶיהָ לָבוֹא אֶל–הַמֶּלֶךְ לְהִתְחַנֶּן–לוֹ וּלְבַקֵּשׁ מִלְּפָנָיו עַל–עַמָּהּ
8. And a copy of the writing that was given to Shushan to destroy them was given to him to show Esther and to tell her, and to command on her to go to the king to mollify him and to plead before him regarding her nation.
The Ibn Ezra points out that Shushan historically had a large concentration of Jews. Therefore, the verse emphasizes that the copies of the posters are from Shushan in order to show the great impact the Jews would feel if they understood the contents of the letters Haman sent.