In planning to write his playJulius Caesar,Shakespeare must have realized that he would need to hire a number of extras to represent the Roman mob, because the funeral oration by Antony and the resulting mutiny were essential. Since he had all these extra men on hand throughout each performance, he may have decided to get some additional use out of them. They serve in the opening scene mainly to illustrate the fact that Julius Caesar is growing in popularity with the masses and represents a threat to the liberties of the upper classes. Shakespeare only provides one important speaking part for the commoners in this scene, and no doubt he used a regular member of his company who specialized in comical parts to play the Cobbler. The exchanges between Murellus and this Cobbler evoke laughter, but they also provide a great deal of information for the audience. Murellus expresses the gathering opposition to Caesar's rise to dictatorial power, a reaction which will ultimately result in Caesar's assassination and everything else that happens in the play. This is summed up in beautiful Shakespearean lines at the very end of the short scene, when Flavius tells Murellus:
Disrobe the images
If you do find them decked with ceremonies.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
These growing feathers plucked from Caesar's wing
Will make him fly an ordinary pitch,
Who else would soar above the view of men
And keep us all in servile fearfulness.
(A short while later, Brutus and Cassius will hear unsettling news from Casca):
I could tell you more news, too. Murellus and Flavius, for pulling scarves off Caesar's images, are put to silence. (Act 1, Scene 2)
Then the mob is used in the famous scene in which Mark Antony's funeral oration turns the tables on Brutus, Cassius and all the other assassins by creating a spectacular mutiny that forces them all to flee the city. This was Shakespeare's main purpose for employing a number of extras to represent a Roman mob. (Act 3, Scene 2)
Finally, Shakespeare inserted a somewhat gratuitous scene in which the mob, undoubtedly composed of the same group of men, encounters Cinna the poet and tears him to pieces only because the poor man happens to have the same name as one of the conspirators (Act 3, Scene 3). Shakespeare gets some extra work out of the extras by this means and strengthens the illusion that the mutiny triggered by Mark Antony's inflammatory speech is continuing and spreading throughout the city.
The mob plays an important role in the play. In the introductory scene we see that there is a political unrest amongst the citizens giving us a feel of the scenario. After the assassination of Caeser, Brutus addresses the mob justifying his actions, thus making him the noble leader of the roman citizens. later mark anthony cunningly leads the fickle-minded mob into a rebel, thus making "the honorable man" into a "traitor". its the mobs behaviour that controls the flow of events.
Firstly the roman mob in the play is fickle minded , they are shifty in character , they could get easily swayed ; this negativity of roman mob was used by mark antony to avenge caesar's death
P.S. This answer of mine points towards the roman mob in the play and no offence to the real world romans
The "mob" is important in the play for the same reason they are important in the history of Rome: The Mob IS Rome.
Gaius Julius Caesar understood this well. He was a popularies, and like the Gracchi before him, he sought to aid the people in return for which the people supported him. Antony, though physically courageous, was not a brilliant leader like Caesar, however he was savvy enough in the politics of the street to understand the power of the 'Mob' and to use their love of Caesar to his own benefit.
For an intriguing examination of Julius Caesar, his assassination, and the power of 'The Mob" see: The Assassination Of Julius Caesar - A People's History Of Ancient Rome by Michael Parenti (c2003)
*Please Note: The video clip from the HBO Rome series is Rated "R" for language and violence. Although fiction, the clip does convey effectively how Antony out manuvered the conspirators.