This seems like it should be somewhat more advanced, because players at a low level often will miss one move threats. But it is basically the first principle from which nearly all other good habits directly derive. Why develop all your pieces, not bring your queen out early, etc. Why do all those things when you know your opponent will often just miss something anyhow if you start pecking him with one move threats?

It's because if your opponent does not miss those threats, then you've likely just made your own position even weaker. And as a corollary a one-move threat which improves your position is perfectly fine of course, but in that case you're not hoping your opponent misses it. I mean you probably are, but you expect him to see it. But it doesn't matter - because your position has improved regardless.