Why are Emilia and Bianca so important in the play Othello?
In additions to the points mstultz72 has made, it is important to consider their functions in the plot or story of the play.
Both do contrast with Desdemona: Emilia's less than ideal relationship with her husband, Iago, contrasts strongly with the deep passion between Othello and Desdemona; while Bianca is spoken about by Cassio as a mere plaything and certainly not wife material in any sense.
But they also serve the plot of the play as characters in their own rights, especially as concerns the event surrounding the handkerchief.
Emilia finds the handkerchief that Desdemona drops and, instead of returning it to her mistress, she gives it to Iago. She then lies to Desdemona,...
claiming she doesn't know where it is. This one action sets into motion the escalation of Othello's jealousy and the supposed proof that Desdemona has been unfaithful with Cassio.
This "proof" comes from Bianca. Iago has given the handkerchief to Cassio, who gives it to Bianca. It is a pivotal turning point in Othello's jealousy when Bianca (while Othello watches in hiding) returns the handkerchief to Cassio. What should Othello think now? Clearly, Desdemona has lied about "having" the handkerchief and has given it to Cassio??!!
This sequence of events surrounding the whereabouts of the handkerchief are key to the development of Othello's "evidence" against Desdemona, and Emilia and Bianca are key players in these events.