The Weird Sisters likely speak in rhyme for a number of reasons. First, it makes them seem otherworldly and strange, as it differentiates their speech from the (usually) unrhymed speech of the other characters. Macbeth and the other nobles speak in blank verse, which is unrhymed iambic pentameter, for much of the play, while the Weird Sisters are the only characters whose speech rhymes with any regularity. It is a sort of aural, linguistic clue that they are different from everyone else—that they are strange.
Second, their rhyming makes their speech sound more chant-like and hypnotic, as though they are constantly casting spells. It fits with what we might expect from witches who conjure spirits, as there is something that feels magical about characters who speak in rhyme. The play opens with this hypnotic speech, alerting the audience to the mystical, magical quality of the action as well as creating a sense of foreboding:
FIRST WITCH. When shall we three meet again?
In thunder, lightning, or in rain?
SECOND WITCH. When the hurly-burly’s done,
When the battle’s lost and won.
THIRD WITCH. That will be ere the set of sun (1.1.1-5).
Third, the predictability of the rhyming, especially because their speech is so rhythmic, seems to add to the sense that the witches do, indeed, know the future. If one is to speak in rhyme, one must constantly think ahead to the next word, phrase, or line, so that it is sure to rhyme with the one just before or after it. For the witches to speak in rhyme so consistently, and even to rhyme with one another at times (as we see above with the second and third witches), means that they certainly do have an eye toward, or even some ability to predict, the future.