Shakespeare  has chosen the witches as characters to represent human tendencies to the evil. In particular, they affect Macbeth’s choices, so they could represent the dark side of his nature. But let’s discuss the pros and cons of Shakespeare’s choice.

The first real problem created by his choice is to understand whether the witches are meant to be independent characters or projections of other character’s hidden personality sides. In the second case, they may distract the reader’s attention from Macbeth’s character and may be seen as mitigating factors to his actions.
This difficulty in defining them leads to different interpretation, depending on whether they are considered to represent the “dark side” of human nature or external factors.

On the other hand, this problem might have been less problematic then we think. Indeed, Shakespeare used the witches as part of the Medieval tradition, and precisely because they were known by everybody in his period as channels for the evil spirits to reach the human beings. Even the illiterate people could recognize them as the trigger to rouse human’s worst attitudes.
To sum up, in the Middle Ages and even more later in the Renaissance, witches were seen as means and pretexts to set off the worst: not mere independent external causes, but opportunities to reveal the worst tendencies of human nature at the higher level.

Moreover, partially transferring the internal struggle between “good and evil” from Macbeth’s mind to the external world gives a lot of advantages. First of all, it is easier to write and imagine for the author. In second place it is easier for the reader/watcher to understand. It is way easier to imagine, follow and understand the interaction between two different real characters instead of trying to convey the same concepts though monologues or other devices.

In conclusion I would say that Shakespeare’s choice to use the witches is supported by both the common character stereotypes of the time and writing/acting/understanding reasons. It was the easiest and more widely understandable way to convey the concept.