What is the purpose of the introduction of cassios mistress bianca with her complaints of being neglected at this point of the play?

Iago calls Cassio in, while Othello hides; Iago speaks to Cassio of Bianca, but Othello, in his disturbed state, believes that Cassio is talking of Desdemona, which is the last "proof" he needs before declaring his wife guilty. Bianca comes in, and gives the handkerchief back to Cassio, since she swears she will have nothing to do with it. She says she is tired of being neglected.