Parenthesis, a final form of hyperbaton, consists of a word, phrase, or whole sentence inserted as an aside in the middle of another sentence:

• But the new calculations--and here we see the value of relying upon up-to-date information--showed that man-powered flight was possible with this design.
• Every time I try to think of a good rhetorical example, I rack my brains but--you guessed--nothing happens.
• As the earthy portion has its origin from earth, the watery from a different element, my breath from one source and my hot and fiery parts from another of their own elsewhere (for nothing comes from nothing, or can return to nothing), so too there must be an origin for the mind. --Marcus Aurelius
• But in whatever respect anyone else is bold (I speak in foolishness), I am just as bold myself. --2 Cor. 11:21b (NASB)

The violence involved in jumping into (or out of) the middle of your sentence to address the reader momentarily about something has a pronounced effect. Parenthesis can be circumscribed either by dashes--they are more dramatic and forceful--or by parentheses (to make your aside less stringent). This device creates the effect of extemporaneity and immediacy: you are relating some fact when suddenly something very important arises, or else you cannot resist an instant comment, so you just stop the sentence and the thought you are on right where they are and insert the fact or comment. The parenthetical form also serves to give some statements a context (stuffed right into the middle of another sentence at the most pertinent point) which they would not have if they had to be written as complete sentences following another sentence. Note that in the first example above the bit of moralizing placed into the sentence appears to be more natural and acceptable than if it were stated separately as a kind of moral conclusion, which was not the purpose or drift of the article.