What is the importance of plotting in Act III, Scene 1 of Macbeth?

The word "plotting" refers to an act in which the perpetrator(s) secretly plan to commit an evil deed. In this sense, then, plotting, by its very nature, seeks a malicious outcome, meaning those who indulge in the act do not wish to do any good and are, consequentially, evil.
Plotting in Act III, Scene 1 is important because it reveals the extent of Macbeth's perfidy. The scene illustrates that he has reached such a state of ruthless and cold-blooded malice, that he has lost all rectitude. There are no boundaries to his pervasive perversion, so much so that he plots the murder of his confidante and friend Banquo and Banquo's son, Fleance.
It is clear from the beginning of the scene that Macbeth has already started plotting Banquo's assassination. Macbeth, who earlier somewhat reluctantly acquiesced to his wife's insistence on killing king Duncan, has now become a master in the art of murder. His conversation with Banquo most pertinently illustrates his sly and wicked intent. He seeks as much information about Banquo's journey as possible so he may perfectly plan the assassination.
When Macbeth meets the assassins, he blatantly lies to them about Banquo's role in their misery and strife, giving them a reason to kill the general. He absolves himself from all blame. Furthermore, he challenges their courage and their will to destroy those who had supposedly done them harm—in this instance, Banquo. They are easily convinced and swear revenge. Macbeth, just to make sure, reminds them that Fleance should also be killed.
Macbeth's cold-blooded statement at the end of the scene most potently indicates the depth and magnitude of his evil:

It is concluded. Banquo, thy soul's flight,If it find heaven, must find it out to-night.

The once-honorable and -admired general has truly lost his way—the fair has become absolutely foul.