Bianca is a fairly minor character in the play, but does serve a few important roles. First, in almost all of Shakespeare's plays, we encounter "rustics" or members of the lower classes. Their first purpose is comic relief. Their second purpose is often to hold up a counterexample to approved behavior in some way.
A central theme of the play is chastity. Desdemona is actually chaste but suspected of having an affair with Cassio. Bianca is actually a prostitute having an affair with Cassio, who ends up falling in love with him. She is a contrast to both Desdemona's naivety and Emilia's cynical conformity to social norms.
The handkerchief belonging to Desdemona that Iago plants and Cassio gives to Bianca becomes key evidence convincing Othello that Desdemona is having an affair with Cassio.