Did you know there are several kinds of what I ubiquitously call a “dash”? Let’s look:
According to the Chicago Manual of Style:
The hyphen connects two things that are intimately related, usually words that function together as a single concept or work together as a joint modifier (e.g., tie-in, toll-free call, two-thirds).
En dashes are also used to connect a prefix to a proper open compound: for example, pre–World War II. In that example, “pre” is connected to the open compound “World War II” and therefore has to do a little extra work (to bridge the space between the two words it modifies—space that cannot be besmirched by hyphens because “World War II” is a proper noun). Now, that is a rather fussy use of the en dash that many people ignore, preferring the hyphen.
Em dashes also substitute for something missing. For example, in a bibliographic list, rather than repeating the same author over and over again, three consecutive em dashes (also known as a 3-em dash) stand in for the author’s name. In interrupted speech, one or two em dashes may be used: “I wasn’t trying to imply——” “Then just what were you trying to do?”
Also, the em dash may serve as a sort of bullet point, as in this to-do list: