Why did Shakespeare add the witches to the play Macbeth?

In introducing supernatural elements into the play, Shakespeare wants to suggest that there's something intrinsically evil and diabolical about disturbing the natural order of things. Macbeth's murderous treachery has upended the political stability of Scotland, unleashing blood-soaked tyranny upon the land. Macbeth, like everyone else, is part of a great chain of being, in which all must perform their specific roles and functions to maintain the stability of the whole. Yet Macbeth's murder of Duncan has disrupted the effective operation of this system, allowing the forces of darkness to prevail.
Macbeth could've had everything he ever wanted by remaining loyal to his king. All of his ambitions and more could've been fulfilled had he done his duty and kept his place in the great chain of being. The problem here is not ambition per se; it's an overweening ambition that destroys the ties of loyalty and duty which bind us all together and which ensure the peace and stability of society. In killing Duncan, Macbeth hasn't just done something morally wrong; he's severed his links with the great chain of being, choosing instead to stand with the forces of darkness and corruption.

There are a few likely reasons why Shakespeare added the witches to Macbeth. One is that the King of England at the time, James I, was both very religious and interested in the supernatural. Adding the mysterious and macabre element of the Wyrd Sisters and their prophecies was likely to intrigue the king, and his patronage was vital to the viability of Shakespeare's career as a playwright.
Another reason to add them is that the witches add complexity to a play that otherwise may simply be thematically concerned with loyalty, manhood, and ambition. Their prophecies create ambiguity, enabling questions regarding what extent their prophecies dictate what happens to Macbeth and others. Would Macbeth, as he speculates in act 1, scene 3, be crowned without him accelerating events by murdering Duncan? It is a question that neither Macbeth nor the audience can conclusively answer, which adds depth and complexity to the plot.