Shakespeare uses imagery for two reasons: it’s Shakespearean special effects, and it creates a mood.
One of the reasons Shakespeare uses so much imagery is that they did not have special effects in those days. The actors had to describe what the audience was supposed to be seeing, if it could not be created. Chances are suspending a bloody knife in mid-air would have been tough with Elizabethan technology. So you just describe it!
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still. (Act 2, Scene 1, p. 27)
This is the same reason Banquo describes the witches beards, and Macbeth talks about Neptune’s ocean not being able to wash the blood off his hands. Shakespeare’s audiences could not see these things on stage, so they had to be described so that the audience could visualize them.
Another reason for the imagery is to create a mood. One of the most vivid and disturbing images is Lady Macbeth’s.
I have given suck, and know(60)
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out (Act 1, Scene 7, p. 24)
This image is terrible, and it confirms that we should not see Lady Macbeth as a mother figure. It also helps to create a mood, as does the witches’ chants. We feel a sense of darkness and foreboding, created by carefully spooky and gruesome imagery.