Elision in Macbeth allows for a few dynamics. First, violent acts appear more grotesque when left to the viewer's/reader's imagination. As the above response notes, many props are required to create a violent murder on stage, and traditional Shakespearean plays use minimal props. So keeping the violence off-stage allows the viewer/reader to imagine the horror. Next, elision allows space for a rapidly advancing plot to develop. Macbeth is one of Shakespeare's shorter tragedies in terms of length; however, there are many events that happen in the timeline of the play. Elision is used in part for efficiency. Finally, elision also keeps the focus of the play on the characters and character development. Viewers/readers may be distracted by violent acts, causing them to get wound up in the gruesome events of the play. But the murder of Duncan, for example, in itself is not the important part--what is important is what leads Macbeth to kill someone whom he respects so deeply. The themes and messages of the play are advanced through character development, so elision helps keep the focus where it belongs.
Shakespeare's use of elision serves two main purposes: it helps maintain the pace of the play, focusing on Macbeth's decline while keeping all of the truly violent action off stage. The first use of elision occurs in Act two, scene two--Duncan's murder. Shakespeare builds suspense all the way up to the deed, as the audience watches Macbeth and Lady Macbeth's crafty plotting of the deed, but the actual murder takes place off stage.
The off-stage murder actually increases the believability of the play's action, because Shakespeare's company would have had a difficult time pulling off Duncan's murder, not having access to many special effects techniques to pull off the disgusting amount of blood involved in Duncan's death scene. Instead, Shakespeare lets the audience relive the moment through Macbeth and Lady Macbeth's point of view, making the murder seem even more grotesque and horrifying.