In act 1, scene 3, the audience is first introduced to Don John, Don Pedro's illegitimate brother, and his plan to swipe at his brother by hurting Claudio and Hero's chance for happiness.
The primary storytelling purpose of the scene is to establish Don John's character. He is as close as Much Ado About Nothing has to a true villain, as his motives are pure malice and mischief. He is envious of Don Pedro's social power and resentful that he must depend upon his good graces. He spells out his hostile attitude in this passage:
I had rather be a canker in a hedge than a rose in his grace; and it better fits my blood to be disdain'd of all than to fashion a carriage to rob love from any: in this, though I cannot be said to be a flattering honest man, it must not be denied but I am a plain-dealing villain.
When Don John learns about Don Pedro's plans to help Hero and Claudio wed, he is excited for the chance to cause his powerful brother trouble. He is a man motivated by anger and envy, so he jumps upon the opportunity to cause problems for the couple if only to exercise some power over other people.
The scene contributes to the main theme of the play through the use of "noting," or overhearing gossip. Borachio shares information about Don Pedro from the court—things he has overheard. Eavesdropping is a running motif throughout the play—particularly eavesdropping that leads to misinformation or deceit. However, here Borachio gives Don John the correct information, yet the irony is that Don John uses his knowledge of the truth to deceive others.