The witches are very descriptive and use imagery to describe their doings.
Imagery is use of language to create a picture in the reader’s mind, or the audience’s mind in this case. Imagery often takes the form of common figurative language like similes and metaphors. A simile compares two things indirectly with “like” or “as” and a metaphor is a more direct comparison, saying something is something else.
An example of imagery can be found in the first witch’s speech. She describes the sailor’s wife munching on chestnuts, which is a descriptive sensory detail that we can see and hear. She also uses a simile.
But in a sieve I'll thither sail,
And, like a rat without a tail,
I'll do, I'll do, and I'll do. (Act 1, Scene 3)
When the witch describes herself as “like a rat without a tail” she is referring to her supposed clandestine travels. She can possibly directly transform herself into a rat also, of course. She is a witch! In this case though, it is a simile because she says “like.” The other witch offers her a "wind" which may be literal or be a metaphor. It may be that witches can conjure wind.
Another simile is used to describe the witches’ influence over Macbeth. The first witch already has a plan for him.
I will drain him dry as hay
Hang upon his pent-house lid;
He shall live a man forbid:
Weary se'nnights nine times nine
Shall he dwindle, peak and pine (Act 1, Scene 3)
The witches are going to deliver the prophecies to Macbeth in order to put him off balance. It is not entirely clear why they have chosen him, but all of their speeches seem to indicate that they desire mischief. They are definitely making mischief in the kingdom by pitting Macbeth against Duncan!
Figurative language adds a lot to a play. Since the witches are a source of mystery and interest, having them talk figuratively and use a lot of imagery helps establish their mystique. The witches are making trouble, and seem to enjoy it. We know that Macbeth is in for a wild ride.