What is the purpose of the following aside? iago (aside) he takes her by the palm. ay, well said,/whisper! with as little a web as this will i ensnare/as great a fly as cassio. ay, smile upon her, do, i/will gyve thee in thine own courtship. you say true,/ 'tis so, indeed./if such tricks as these strip you out of your/ lieutenantry, it had been better you had not kissed/ your three fingers so oft, which now again you are/ most apt to play the sir in. very good, well kissed,/ and excellent courtesy! ’tis so, indeed. yet again/ your fingers to your lips? would they were clyster-pipes for your sake! —[trumpet within}. the moor! i know his trumpet. (act ii, scene 1, lines 165-176)

The following aside of Iago shows him planning to plant Cassio and Desdemona to ruin not only Othello but also at the same time ruin Cassio and his reputation. He is willing to do anything as long as it will be for his own good. His deceptive nature to even ruin Cassio is fully shown here.

Explanation:

William Shakespeare's tragic romance "Othello" tells how a trusted friend and companion of Othello bring about the downfall of everyone, even leading to a  tragic death of the characters. One of the scheming ideas of Iago is seen in Act II scene 1 where Cassio and Desdemona had met with Iago's plan.

Cassio had been in love with Desdemona but she did not know about that. Iago had wanted to ruin not only Othello, Desdemona's husband for marrying a white woman while he is a black moor. But he also wanted to ruin Cassio's reputation and maybe be stripped of his position in the army and that post may be given to him (Iago). So, he had agreed to help Cassio to win Desdemona's love, manipulating him in believing that Desdemona will eventually get tired of her black husband and will enable Cassio to be with her. This aside(a hidden thought spoken out by the character but not in a loud voice) of Act II shows the manipulating and cunning plan of Iago. He is happy to see that Cassio is doing as he had planned, which will make it easier for him to ruin him. This aside also serves as a chance to show the deceptive nature of Iago, willing to step on anyone as long as it is going to be in his favor.