Example 1
All chickenburgers are created equal—at least that's what I thought before I saw Food, Inc., a documentary about America's corporate controlled food industry.
In this example, the em dash is used to show a sudden change in thought. That is, the speaker originally didn't put much thought into where their food comes from. After they saw the documentary, their view changed. Their eating habits probably did, too.
Example 2
My boss—the most fashionable woman I've ever seen—broke both of her arms in a bar fight in Tijuana last weekend.
Although we have several questions about the speaker's boss after reading this sentence, the fact that she has an exceptional sense of style isn't essential to understanding it, so em dashes are used to quarantine that chunk of nonessential information.