In act 3, scene 1, why is the aside from Cassius to Brutus before Antony's speech important?

In Shakespeare's Julius Caesar, the
aside when Cassius talks to Brutus about not letting Antony speak to the crowd at
Caesar's funeral is important, because it provides an example of Brutus's poor judgment,
which is what leads Brutus, the tragic figure in the play, to his
fall.

Cassius tells
Brutus:

You
know not what you do.  Do not consent

That Antony speak in
his funeral.

Know you how much the people may be
moved

By that which he will
utter?

But Brutus decides to
let Antony speak anyway.  Of course, we know what happens.  Brutus errs in many ways. 
First, he underestimates Antony.  Second, he overestimates the importance of his
announcement to the crowd that Antony speaks only with his permission.  Third, he
underestimates the fickleness of the Roman crowd.  Fourth, he thinks the reasons behind
the assassination of the crowd will be evident to and accepted by the crowd without
question (and they are, until Antony speaks).  Fifth, he doesn't consider the
possiblility that Antony will speak with irony, thereby fulfilling his promise to not
speak against the conspirators, while still turning the crowd against them (this last
one is a bit of a stretch, I know).

Cassius, as it turns
out, is a much better judge of people and situations, and a much better decision maker
than Brutus.  Though Brutus is the noble one of the two, the conspiracy may have
succeeded if Cassius were the one making the decisions.