Throughout the Megillah, the term משתה is consistently used to refer to feasts (ex. 1:3,5,9; 2:18). Rashi on Esther 5:4 writes:
אֶל הַמִּשְׁתֶּה. כָּל סְעוּדָה נִקְרֵאת עַל שֵׁם הַיַּיִן שֶׁהוּא עִיקָר:
"To the משתה" – any feast is called after the wine, which is the primary part.
(Seemingly Rashi is addressing the usage of משתה, from the root שתה, to drink.)
The term appears nineteen times in the Megillah. But in three of them, 7:2,7, and 8, all in the context of Esther's second banquet, the Megillah instead refers to it as a משתה היין, a feast of wine.
If the term משתה already implies that the main part of the feast was the wine, why does the Megillah add the term היין, of wine, in these three pesukim?