On a simple level, the verse tells us of Esther’s beauty to show that she seemingly fits into the king’s wife-search criteria. In fact, “her beautiful form” was a detail the king’s adviser’s do not mention. The Talmud (Megillah 13a) tells us that one aspect that attracted her to everybody was that she looked to everyone as though she were from each person’s group.
Perhaps this is a reference to “ishas yifas to’ar” (“woman of beautiful form”) whose appearance seduces the Jewish soldier when he is in his weakest spiritual condition (Devarim 21:10-14). Such a woman can be taken even if she is already married (Talmud, Kiddushin 21b), much like Esther was.