In Act 4 scene 3 Malcolm is trying to see if he can trust Macduff (4.3.178-243). Macduff asks about his family and Malcolm conceals the truth about his family and tells him they are well when in reality Macduff’s family has been murdered. Once Malcolm knows that he can trust Macduff, he tells Macduff about how his family has been killed and how he should use his anger that he feels into killing Macbeth. Malcolm has already created an army with 10,000 men ready to go to Dunsinane to kill Macbeth. Macduff agrees to do that and they head that way. If Malcolm had told Macduff in the beginning that his family was dead, Macduff could have been less willing to help kill Macbeth.
Another reasons for equivocation is you can make alliteration to occur which can help with sound devices and allow for sound devices to happen (Visual Thesaurus). For example in Act 1 scene 1 line 10-11 the witches say “Fair is foul and foul is fair, / Hover through the fog and filthy air”, some other words could have been placed here and the same meaning would come through (1.1.10-11). All the “f’s” could have been changed to other constants but the sound “fff” would not have had the effect that it