Bubble Outs are among the most useful of all trial exhibits. They highlight an area of concern and deliver it to decision makers in a way that is clear and totally un-ambiguous. They allow the attorney to direct the jurors' attention to the specific point at hand, without worrying that the jurors are examining other parts of the document such as the date, or the answers to other questions on the page.
Another example of a bubble out exhibit that compares what this doctor was paid for a particular procedure, and what he charged the insurance company (twice). It is pretty clear there is something fishy going on here.