I use it mostly two ways. Important emphasis enclosed statement as compared to in between parentheses which I treat as lesser required context/info. Second way is an indicator of a pause in a statement but not so much like an ellipses. Like a short pause for a punchline whereas ellipses for a long thought or time collect feelings/compose oneself. A sharp contrast compared to a period from the first part of the sentence to the post-em dash part of the sentence. I’ve been using it before LLMs and frequent enough that I am pretty self conscious now since I’ve noticed people call out em dashes as a call-sign for bots. A lot of times it’s such an innocuous usage that I feel like people are witch-hunty paranoid reading posts on the internet