'Know this'

Here’s one of my pet peeves about television writing. It seems that a writer wants to show a character getting serious and reinforcing some sombre truth, she has that character say to another “know this: I will never leave you alone” or “know this: I’ll be watching you” or something like that.

This always grates on me. Nobody talks that way. It’s not as if it’s unintelligible, but it’s so writerly that it always distracts me. Real people would say something like “I want to be clear about this” or even “You should know this one thing…”

Why do they do it?