Emilia does serve to further the plot as a means to get the handkerchief to Iago, discover Desdemona's murder, etc., but her role is much more important.  Emilia (and Bianca to some extent) serves as a foil to Desdemona in the play.  Throughout the play Emilia demonstrates her sharp, witty tongue.  She is a woman who has no problem discussing her willingness to have an affair.  "let husbands know their wives have sense like them; they see and smell and have their palates for both sweet and sour, as husbands have." (IV, iii, 99-102)  Desdemona, on the other hand, always declares her faithfulness to her husband and to her marriage.  Emilia distrusts her husband and marriage in general.  She believes that men and women are basically the same and have the same desires and needs.  While Desdemona's character reflects the typical Renaissance view of relationships between men and women, Emilia represents a progressive attitude toward the role of women and women's abilities to have feelings, and desires that have nothing to do with men.  ("And have not we affections, desires for sport, and frailty, as men have?"  IV, iii 105-106)    Emilia's fate is parallel to Desdemona's as well. Both die "guiltless" deaths at the hands of their husbands. 

I'll let you work on expanding them, but here are some functions:

• She gives the handkerchief dropped by Desdemona to Iago
• She is relentless in trying to find out who killed Desdemona and bring Othello to justice
• She exposes Iago about the handkerchief
• She is the only character Iago cannot control or manipulate