Just so you know

So why do people think by saying "just so you know" at the end of the sentence makes people aware of whatever they are telling you.

In some cases, that can be true. Maybe I don't know the fact you just told me.

But what if I already knew that fact. Adding "just so you know" just infuriates me to no end. Yes I know "the t.v. is on" cause I'm watching it or that "the door is still open" cause i can hear the street traffic or feel the breeze.

So what if I already knew that if I was the last one watching TV, I would turn it off, or the last one out the door, that I would close and lock the door.

I feel that when someone adds "just so you know", you are implying that I didn't know the fact you are talking about and that I was really that stupid that I didn't know, that I was oblivious to my surroundings or that i just didn't care.