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.