Trochaic meter (consisting of singular trochees) is the exact opposite of iambic meter: trochaic meter a metrical foot made up of one stressed syllable followed by one unstressed syllable. Conversely, iambic meter is made up of one unstressed syllable followed by a stressed one. As a result, this puts emphasis on the beginning of the words in the line:
DOUble, DOUble, TOIL and TROUble.
FIre BURN and CAULdron BUbble.
This haunting 'loud-then-quiet' effect acts to intimidate the audience and gain attention to what they are saying as the first syllables are weighted, with the end syllables (perhaps less important) melting away. You may have noted a similar device at the beginning of many of Shakespeare's plays where they open with a loud scene such as a fight (like in Romeo and Juliet) or a shipwreck (like in The Tempest) or even the three scary witches from Macbeth!
Contextually, this was because the crowds were often rowdy and so a loud opening to a play was essential in arresting the audience's attention. This mainly creates distinct chanting rhythm that characterises the witches' speech and aims to evoke a sinister atmosphere, as oppose to the poetic, songlike iambic meter - the trochaic meter is almost hypnotising. It sets the scene (reference 1) (reference 2)
Also, the Elizabethans had a preoccupation with the supernatural and there is a pattern between trochaic meter and supernatural beings in his plays (reference). Not only is it used by the witches, but also the fairies in A Midsummer Night's Dream; here, Puck addresses Oberon, King of the Fairies:
CAPtain OF our FAIry BAND,
HELeNA is HERE at HAND
(this programme is very good if you're interested).
So overall, the use of trochaic meter adds a chanting, arresting quality to the witches' spells as well as separating them from the mortal world who mainly speak in iambic pentameter in the instances when they speak in verse or blank verse.