One purpose that this scene serves is to show that Banquo is a threat to Macbeth's plans. This relationship is complicated. One one hand, they are friends. In fact, when Banquo questions who is approaching him in the darkness, this is how Macbeth identifies himself.

Yet the tension is growing here. They meet in darkness, serving as a metaphor for Macbeth's current state of mind, and Banquo's first reaction when he hears someone approaching is to draw his sword. This physical reaction is not unwarranted considering Macbeth's plans.

The beginning of this scene also serves as a reminder that Banquo is a threat. He knows of the prophesies of the "three weird sisters," which prophesy both Macbeth's rise to become King. The audience realizes that Banquo knows too much. If Duncan is murdered, Banquo could become suspicious. The mention of the three weird sisters also serves as another reminder: Banquo will never become King, but his descendants will. Banquo's mention of the prophesies sets the stage, foreshadowing that Macbeth's hand will be in the machinations of Banquo's demise.

Banquo shows a bit of suspicion in this scene, telling Macbeth,

So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counselled. (II.i.32-35)

Banquo's desire to maintain a clear conscience and to remain loyal in his allegiance to Duncan makes him an eventual enemy of Macbeth's own plans. Here he tells Macbeth that he will listen to his counsel as long as it doesn't violate his own code of honor. The audience realizes the dramatic irony of this situation.

So I lose none
In seeking to augment it, but still keep My bosom franchised and allegiance clear,
I shall be counselled.
(act 2, scene 1, lines 33–35)

The conversation between Banquo and Macbeth gives the audience further opportunity to understand the friends' now-diverging characters.  At first, both Macbeth and Banquo were noble and loyal (consider the captain's description of Macbeth in Act I); now, however, we know that Macbeth is plotting Duncan's murder, and this scene shows us that he is even willing to lie to his best friend.  In regard to the Weird Sisters, Macbeth tells Banquo, "I think not of them" (2.1.27-28). Lies: he's been doing little except thinking of them.

Furthermore, when Macbeth asks Banquo to "cleave to [his] consent" when Macbeth asks, a loyalty which he claims will "make honor for [Banquo]," the loyal and forthright Banquo responds, "So I lose [no honor] / In seeking to augment it, but still keep / My bosom franchised and allegiance clear, / I shall be counseled" (2.1.36-39).  Such a response accomplishes two things: first, it shows that Banquo is truly honorable, and, second, it shows that Banquo may be developing some suspicions of Macbeth.  Why would Banquo assume that Macbeth would ask him to do something that would go against his conscience unless he already suspected that Macbeth were capable of such action himself?

This short scene shows some characterization of both Macbeth and Banquo.  Macbeth asks for Banquo's support when the time comes without telling him why which shows the importance of this alliance is to Macbeth.  Banquo replies by saying that he will support Banquo with anything as long as his (Banquo's) conscience can remain clear and he won't lose any honor because of his actions.  This should show the cautious support of Macbeth given by Banquo, but also that Banquo is beginning to suspect the Macbeth has poor intentions in mind.  Both of these men have been thinking about the prophecies quite a bit.  Macbeth is inclined to take action, but Banquo would prefer to wait and see what happens.