It's a detail that isn't absolutely essential, but is very helpful to describe something. That something can be a relationship, or reasoning behind something, or whatever it is you want to explain at that point.
As Ernest stated, such details are rarely "nonessetnial". Instead, they more often foreshadow future events, and thus you (the author) want readers to notice them. Instead or being 'non-essential', they are outside the main scope of the sentence, and thus you separate them from the main sentence so the idea will stand on it's own (in relation to the larger sentence).
Essentially, the statement wouldn't (or shouldn't) be made if it wasn't essential to the story, and it wouldn't be added to the sentence if it didn't immediately relate to the rest of the sentence. However, that doesn't mean they should be excluded either. How much you want to emphasize it (i.e. whether it warrants an em-dash or not) is ultimately up to each author.