Answer and explanation:
The purpose of using dashes in this excerpt is to set off an appositive sentence that contains commas. An appositive sentence is a structure that offers further information on something mentioned before it. Appositives can usually be set off by commas but, when they are long and present commas in their own structure, they are set off by dashes.
In the excerpt we are studying here, the appositive "-so-called because they used to, like, DECLARE things to be true, okay, as opposed to other things are, like, totally, you know, not-" is a very long one and full of commas. It is supposed to offer further information on what declarative sentences are.
The Mongols spread the use of gunpowder.The Mongols controlled both China and Russia.<span>Pax Mongolica encouraged trade along the Silk Road. I would say these are the main effects of the Mongol incursion into Europe and Asia plus the inadvertent introduction of the bubonic plague of infected rats from Asia to Europe.</span>
