He actually thinks these things since he’s just a rotten human being
He does this in order to foster a relationship between Desdemona and Cassio, so that they gain some rapport with each other
He does this in order to foster a relationship between Cassio and Emilia, since he wants to get out of his marriage to Emilia so badly but the only way to do so is if he catches Emilia cheating on him with Cassio (or any other man)
He does this in order to publicly shame his wife, who he hates