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.