Two words, a sentence. So many people triggered! I've had to completely ban this sentence from my vocabulary because people felt attacked by it. So, let me explain what "I know" means for me.
This is a sentence I've used a lot when I was younger, when people were trying to impart me some random wisdom and expected me to be glad they did. Thing is, this often came with the fact that they never intended to have a discussion about said topic and only went on on what they THOUGHT I knew, based on my resulting choices. Why? Simply because my choices were different from what they EXPECTED me to choose, had I known the same thing they did. After all, it just makes sense to make the same choices when you have an educated answer to a problem, right? Except, life ain't basic math.
In no way, do I HAVE to make the same choices you made, based on the same knowledge. Maybe I have MORE knowledge and research than what you do, maybe the answer you've reached for yourself just doesn't fit in with my own reality. You're just forcing your bias on me.
So, I know, means: I am aware of everything you just said, and I still decided on a different answer than yours simply because my life is different than yours. It does not mean: I wasn't aware but I prefer to completely ignore everything you just said just to piss you off. However, it DOES mean: I don't know why I would keep having that discussion with you since, clearly, you don't intent to listen to my own input about my own problem.
"But giving people advice and wisdom is just a normal thing, stop being affected by it, they mean well." you might say. But, it being normal and expected, does not make it less annoying or bad for others, and it doesn't mean I should stop fighting for change, just like ANY marginalized group. Sure, you don't see any physical impact short term from such behavior, but that doesn't mean it doesn't leave mental traces. It doesn't mean someone won't be bullied or shunned for their difference. So I'll keep speaking from personal experiences.
So the next time someone answers you "I know", please ask yourself if you're really trying to help the person, or if you're being self-important with what you see as wisdom and your own view of reality.