How does Desdemona kind concern for Cassio plight unwrittingly play into Iago hands?

Iago's plan hinges on Othello's blind jealousy for his Desdemona. Iago sets up his plan to ensnare yet witless young Cassio. Iago plants the seed in Othello's head that there could be an attraction between Cassio and Desdemona. Desdemona's kindly meant pleas to Othello on Cassio's behalf plays right into Iago's fabrication that the two are attracted to each other.