Answer:
Iago sets out to trap Othello by several means. He persuades Desdemona to plead for Cassio and persuades Othello that Desdemona pleading for Cassio proves she is in love with him. He steals the handkerchief, persuades Othello that she has given it away to Cassio, then gives it to Cassio himself. He gets lucky as Cassio gives it to Bianca and she returns it in Othello's sight. He talks to Cassio about Bianca, telling Othello who can see (but not hear) that he is talking about Desdemona. All of this circumstantial evidence bolsters the outright lies that Iago tells about Cassio admitting to having an affair with Desdemona.