Why does Shakespeare use the technique of elision in Macbeth, in which certain key events take place offstage?
Your question regarding off stage action in Shakespeare's Macbeth is an interesting one. It might be useful first of all to consider which actions are completed off stage: the slaying of Macdonwald, the execution of Cawdor, the murder of Duncan, the murder of Lady Macduff, Lady Macbeth's suicide, and Macbeth's death. In other words, most of the violence in this very bloody play takes place offstage, and this action is reported by other characters. Logistically speaking, Shakespeare most likely had to omit some of these scenes because they would have been very difficult to stage. His ability to use special effects was quite limited. Further, it is quite likely that Shakespeare was influenced by the Greek tragedies of Sophocles and Euripides who also consigned violent acts to offstage, probably to maintain the three unities of time, place, and action.
Nevertheless, some of the reported action is more horrible in the imagining than in seeing it performed on stage. Macbeth unseaming Macdonwald from his "nave to his chops" is one such example. The execution of Cawdor is interpreted for us. In other words, from the report of this death, we understand that Cawdor died nobly.
Nothing in his life became it like the leaving it.
And these words become quite haunting when we see Macbeth meet his own death quite courageously. Both men, though, are traitors.
Yet, some gruesome or violent scenes are portrayed on stage: the return of Banquo's ghost, the severed head of Macbeth. Macduff's son is stabbed in full view of the audience--this act which takes place in broad daylight serves to highlight the depths to which Macbeth has sunk. No summary dialogue or interpretative words are needed here, for the poignancy of this brutal act is best captured by the child's words:
He has killed me, mother.
So obviously it was not just logistics dictating the mode of the action. In many cases, it seems to be the effect that Shakespeare desired to achieve. We don't see Lady Macbeth's suicide perhaps because Shakespeare wanted to focus more on Macbeth's numb response. We don't see Lady Macduff's murder but we hear the screams offstage and imagine the worst. We don't see the actual beheading of Macbeth, but we do see Macduff holding Macbeth's head high in victory amid cheers of joy.