They are easily swayed, veering from supporting one character to the next after Caesar's murder. After the death of Caesar, for example they hail Brutus as a hero. Then Antony's speech, while ostensibly only done to "bury Caesar," persuades them to support him, and they run riot in the city, chasing the conspirators into exile. So they are the force behind the actions of these men, and they are very easily harnessed in support of one man or another. A bit of a cynical view of urban crowds, but it does leave us asking who is supposed to be the "good guy" in the play, which is one of its many qualities.