The opening of any Shakespeare play, including Macbeth, is about exposition.
In the short opening scene, Act 1.1, the following is revealed:
- The weather, thunder and lightning. This is not a "sunny" play.
- The witches: the supernatural will be prevalent.
- The witches will meet again in "thunder, lightning, or in rain?" Again, sunshine is not an option.
- A battle is going on--the hurly-burly. Definitely necessary information.
- They'll be meeting on a "heath," which has negative, wild connotations.
- They'll be meeting someone named "Macbeth."
- The witches call on spirits for help.
- Things are not as they seem or are supposed to be: what's fair is actually foul and what is foul is actually fair.
- The air is foggy and filthy.
The mood and atmosphere are set for the play here, as well as the main character and dominant themes introduced.
Concerning why witches are used instead of any other means, any answer is partly speculation. We can't go back and read Shakespeare's mind.
At the same time, the effects of using witches are evident. In addition to the list of what the witches reveal, we know that Macbeth will later be associated with the witches when he echoes the fair and foul line, and we know that the witches serve as the catalyst for the plot and the conflict. It is the predictions the witches make that ignite Macbeth on his course of action.
And, by the way, James I, the reigning monarch at the time the play was first produced, was fascinated by witches. That may have influenced Shakespeare a bit.