Why is it said that your knees should be below your waist while sitting and meditating?

I am always told that meditation is meant to teach you how to let go of thoughts, and most guided meditations tell you to let go of thoughts, but what does this actually mean, and how do you actually do it?

I heard there are some meditations that you meditate with your eyes open. What are the benefits of this compared to meditating with eyes closed?

Not that you are necessarily dreaming of anything, but just the feeling as if you are in a dream. Everything is sort of slowed down, and you don't really have too many thoughts anymore. Yet everything also feels magnified.

How did that session impact your mind, mood, your day, and week? It would be interesting to know the impact long sessions have on people.

I realize that I tend to force things in life, specifically situations, and this is something that I am trying to stop. But, I am now uncertain as to when I am forcing things compared to when I am initiating things. Doing things, such as approaching people, is not always forcing it, but I'm just having a hard time realizing the difference.

Guided meditations usually instructs you to bring your attention or awareness back to the breath when you notice you are thinking, but what good does that actually do? Why the breath specifically?

This is directed at beginners but also experienced meditators, really to anyone who has noticed changes in their lives that they relate to meditation. I want to know what those changes you notice are.