Since witches were indicative of evil and the supernatural in Shakespeare's day, having them introduce the first scene of the play starts the play with a sense of foreboding that something sinister and/or evil will occur. They agree to meet again on the heath when the battle is over. They expressly note that they will meet with Macbeth. Given that the witches represent evil, their upcoming meeting with Macbeth suggests that the supernatural world and the world of humans (Macbeth) will interact. Particularly, the witches will have some impact on Macbeth himself. The final lines blatantly indicate that bad things will happen.
Fair is foul, and foul is fair.
Hover through the fog and filthy air. (I.i.11-12)
In the next scene, we learn that Macbeth has fought bravely, a loyal servant to the king. At that point, Macbeth is described in good terms. But as the witches indicated that they will meet with Macbeth (in Scene Three), and as they indicated that what was good will become evil ("Fair is foul"), they allude, at least to the potential, that their upcoming meeting with Macbeth will have to do with a transition from good ("fair") to evil ("foul").
The first scene of Macbeth serves a number of important functions. First of all, it helps to establish a dark and tense mood. This is primarily achieved through the scene's stormy setting ("thunder and lightning") but also through the supernatural elements, most notably the three witches and the appearance of their familiars.
This scene is also important because it introduces an element of mystery to the plot. The third witch, for example, mentions a meeting with Macbeth but does not indicate any reason for a meeting. This successfully piques the reader's interest by implying that Macbeth is a character of interest and significance.
Finally, this scene encourages the reader to think carefully about what lies ahead. When the witches say, "Fair is foul, and foul is fair," for instance, they are warning the reader that all is not as it seems.