Those who perform such good deeds, even though they are normally done in public, should do so modestly, without excess or attracting undue attention to themselves. Queen Esther is praised for keeping the secret of her Jewish identity until the right time came to divulge it. In the Jewish tradition of anonymous authorship, his goal was not fame or honor for himself, but to share his wisdom with others. His own self-effacing attitude is the eptiome of modesty. This, then, is the Jewish concept of tsniut , modesty.
Modest attire is but one manifestation of it.
The role of the dress code is that it serves as a way of de-emphasizing the physical self. It helps us all, men and women, to think about each other and ourselves as human beings with a spiritual dimension, and not just as creatures of desire. Outwardly, one should rejoice normally along with everyone else, participating fully in the singing, dancing and feasting. Modesty consists in striving to direct all of those activities for the happiness of the wedding couple, not for oneself. Any excess, anything not necessary for the mitzvah goal, should be put aside.