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.
