A comma is not the only way of dividing up a long sentence. Brackets and dashes let you pull out a phrase so it can be read apart from the main thought, making the whole sentence easier to unload. Bracketing a phrase off puts less stress on it than parenthesizing it between commas; dashes put more stress on it. Imagine reading the sentence aloud. You drop your voice to bridge over words in brackets (to signal that this is a slightly sotto voce sideshow to the main thought) and then resume a normal tone. But you raise your voice when bridging over words in dashes – like this bit that I want to
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