When it comes to writing, all information is not equal: some of it is given as an aside—not essential, but interesting, useful, or “nice to know.” How do you mark this kind of information? The most common, and least obtrusive, way is to enclose such information within a pair of commas, as in “Potassium cyanide, which has a characteristic bitter-almond smell, is a deadly poison.” Grammarians call the extra information a “non-restrictive clause” and the pair “non-defining commas.” But that is another post.
However, there are two other devices, namely enclosing the extra information within brackets (parentheses) or enclosing it—like this—with a pair of em dashes.
Now, how does one choose among the three alternatives? Well, the choice depends on the effect that you as a writer want to achieve or your assessment of the value of that extra information.
1. Use commas when you do not want to disrupt the flow. A pair of commas encloses the extra information with least distraction: the extra info blends slickly with the rest of the text.
Two. Use brackets when the information is purely incidental and unimportant but may help some readers. A typical use is to define a quantity in an alternative unit of measurement, as in “The car was traveling at 60 kilometers (harshly 37 miles) per hour.”
Trio. Use em dashes to emphasize or highlight the information, as in “It is possible—indeed very likely—that biotechnology will bring in prosperity for farmers.”
Read this article to understand the comma usage better: Scientific writing: Comma usage in “if-then” sentences.