Why do you think shakespeare made opening scene featuring the three witches?

Not all film versions of the play start and end with the witches. Orson Welles' 1948 film does, in order to reinforce the idea that the witches totally controlled Macbeth and caused him to do everything he did. In this film he was their plaything. Roman Polanski's 1971 film ends with Donalbain seeking out the witches to get help against his brother Malcolm. In Polanski's film the witches represent less a dominating force but rather a resource of evil to which human beings are drawn in their lust for power. Polanski is making the point that the evil tendencies recur inevitably.