Some scholars believe that the character of Hecate and the scenes in which she appears were added to Shakespeare's Macbeth by Thomas Middleton (1580–1627) or another playwright at some time after Shakespeare's death. It's for this reason, and for the generally subpar quality of the writing, that this scene is often omitted from modern productions of the play.
Macbeth mentions Hecate in two lines prior to her entrance in the play—"witchcraft celebrates / Pale Hecate's offerings" (2.1.59–60) and "ere to black Hecate's summons" (3.2.45)—but these are passing references with no particularly significance in the play.
In act 3, scene 5, Hecate meets with the three Witches and angrily berates them for daring to "To trade and traffic with Macbeth / In riddles and affairs of death" without her.
Hecate refers to herself as "the mistress of your charms. / The close contriver of all harms" (3.5.6–7), meaning that she's their superior and she's the one who should be organizing the plotting...
(The entire section contains 4 answers and 1392 words.)