Social engineering (SE) is not only about exploiting information which attacker has, but also about exploiting patterns of (human) behaviour.
To explain this, let's do a little exercise - say out loud the color, not the word.
Can you see the "exploit" here? The use in real life situation of this "exploit" is very questionable, but it very clearly shows us how our brain can be manipulated even if we have the valid information (we all learnt colors when we were babies).
The real life example could be something like this - let's say you want secretery to put your USB into her machine. Going to her and polite asking her to do so might be rejected, especially if there are policies which forbids this. But you could suit up, spill coffee on your shirt / trousers and on your papers and then come to her, holding those papers and saying - "I'm so late to the meeting and while I was driving to here, cat ran out infornt of my car and I started breaking really hard. The cat did survive, but my papers didn't. I know this is strange request, but please, could you print it for me? I'm really late and your boss might be really angry at me!"
This is called pretext and basically, it's a role played by SEr. What are we doing in this pretext? We are exploiting emotions. If this is played well, and your microexpressions are genuine, most likely she'll do what you want. Why? Because we, humans, are codded like this. Yes, she might know that putting unknown device in her PC might be harmful; yes, she might be educated about it, but let's be serious, you tried not to hit the cat, you didn't drink your coffee, you ruined your suit, you're late on meeting, boss will be angry on you, and now some policy asks her to be rude to you. Come on...
However, key part here is to set her in the right mood - to feel sorry for you. To do so, your microexpressions must be interpreted as true (genuine) by her. If you played your cards right, you have the same effects as with colors. She knows it's something she shouldn't do it (color of words), but emotions are telling her otherwise (meaning of words).
Another trick which SEr can pull on target is, so called, Pavlov's dog experment. So, what does the droolling dog has to do with ITSec? Let's say I want to know about physical security at your workplace. You know you shouldn't share that information with me. I also know that after work, you always come to local pub for a drink. One day I introduce myself and we start small-talk. At first it was just about your cool car. Then we started to talk about women in bar, then about our exs, about last year vacations and so on... All in all, something what is not unusal to talk about, but it's from private life. When we met, you noticed that everytime I ask question I hit table with the cigarette. At first it might be even annoying habbit, but then you just ignored it. After few days / weeks when you started to feel comfortable around me, I started to ask about your work and work environment. And bit by bit, you told me what I wanted to know about physical security in your company.
So what did I do here? By casual talking to you, I trained your brain to give me answers everytime I hit table with cigarette. While this is not brain-washing, and by just doing so you wouldn't tell me your darkest secrets, imagine this as - pilling one layer of onion. The second layer was trust I gained with time spent with you in bar. And so on and on... I did manipulated you and this simple trick helped me to not raise any red flags when I asked you sensitive questions. Again, it wasn't about information you have (do not tell that to strangers), but about your behaviour and reaction to outside world.
What I'm trying to say here is - no metter what you know, if you are placed in right situation, you'll do what is asked from you. Why? Because it's in our genetics.
Just to give one or two "out-of-IT-sectore" examples how information / knowledge which target has can be meaningless if he/she is attacked by skillful SEr. In court, evidences are pure cold facts, yet, good lawyer can, no metter in how bad position his client is, turn those facts in his favor using SE.
Before you are buying car, you'll go and inform yourself which is the best for you. When you arrive at shop to buy one, seller can convice you that you should buy more expensive car, again, using SE.
Also, check this video. How he did it? By just acting normal. Nothing more.