Why does Cassio summon othello?

Othello begins on a street in Venice, in the midst of an argument between Roderigo and Iago. The rich Roderigo has been paying Iago to help him in his suit to Desdemona, but he has seen no progress, and he has just learned that Desdemona has married Othello, a general whom Iago serves as ensign. Iago reassures Roderigo that he hates Othello. Chief among Iago’s reasons for this hatred is Othello’s recent promotion of Michael Cassio to the post of lieutenant. In spite of Iago’s service in battle and the recommendation of three “great ones” of the city, Othello chose to give the position to a man with no experience leading men in battle. As he waits for an opportunity to further his own self-interest, Iago only pretends to serve Othello.

Explanation:

Cassio is a man who is loyal to his general, Othello, but despite his loyalty is believed to be cheating with Othello's wife, Desdemona because of his good looks, his personality, and his friendly nature with women. He is popular and has a good reputation until he is used by Iago to destroy Othello.