In chemistry, a pair of dashes can be used to enclose necessary information, particularly when a sentence is long. Usually, commas are enough, but in sentences that already contain sub-clauses (phrases that supply more information about the main subject of a sentence) dashes sometimes help to keep the meaning of the sentence clear.

Good example of use of dashes:

This sentence is long, and the dashes help the reader to follow the sense of the sentence.

However, essays littered with unnecessary dashes irritate readers.

Poor example of use of dashes:

Here, we have only one sub-clause: belonging to the monoamine oxidase inhibitor class of antidepressant. In this case, dashes are too strong and commas would be much more appropriate.
A tip: commas and dashes are not interchangeable. Commas will nearly always be more appropriate. If you've more than four pairs of dashes on a page, check to see that they really are necessary.