An em dash marks the dramatic entrance of another related element in the sentence, or an authoritative sectioning off of nonessential sentence elements.
Basically, commas, colons, and parentheses walk onto the stage peacefully. Em dashes run onto the stage with spirit fingers blazing. Because they like to make a scene, only use em dashes when you purposefully want to interrupt the flow of a statement and prepare the reader for something important, or if you want to firmly fence off supplementary segments from the rest of the sentence.