The witches use 'Hail' to greet Macbeth and Banquo as a form of yelling or calling out to them.
Hail in the context of how the witches use it refers to a. to yell for or to call. The witches are shouting out 'Hail' to greet Macbeth and Banquo, not trick or fool them, salute or flood them with rain.
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