Let us appreciate the singular quality of the em dash, my favorite punctuation mark and one that is entirely underused. I am not speaking of the paltry hyphen, which is only useful for connecting words and dates—but the em dash, more than twice the length and with quite a bit more emphatic heft. Observe how it strikes through the previous sentence, drawing the reader's attention to its clause in a way that commas and semicolons simply cannot.
Don't mistake me; semicolons and commas are quite lovely, and I always appreciate their proper usage—but the em dash, to reference Wilbers Mastering the Craft of Writing, is just so dashing. Type two hyphens between phrases and voila, Word knows exactly the character to substitute. Named for being approximately the same length as the "m" character, it spotlights clauses, appositive, and other sentence elements without adding a word.
Commas perform only the most perfunctory separation, whereas parentheses practically make the enclosed information invisible; and semicolons provide a pause slightly more brief than a period. Colons (hardly employed more often than the em dash) serve to introduce: This pertains to that. To drive a point home—to capture your readers—add a dash or two.