Q1. Why zazen？
A1. There can be different purposes for sitting zazen, depending on individuals. If a person is mentally and physically exhausted, living hectic lives, zazen could be a way to get relaxed. One could sit out of that person's own interest, or sit in order to train oneself in some ways; one could be interested in the cultural or historical background of zen practice. That said, the primary aim of sitting zazen is ① to realize one's own true nature. ② To make use of that realization in your daily life, and ③ To get rid of one's attachment to (the experience of) realization. To sum up the three above, the primary aim of zazen is to thoroughly embody the realization of one's own true nature.
Q2. What is "satori (self-realization)"?
A2. The content, if any, of self-realization can't be described directly, even with the use of the most sophisticated language that has ever existed. So, for the sake of answering the question, I'd like to change the question itself into "What happens when one realizes one's true nature?" To that question, the answer would be "with the realization, one is crystal clear about who s/he really is."
Q3. Why should one realize his / her own true nature？
A3. It's not that every single person should realize it in a hurry, nor is it that one decides to do so because s/he was forced or persuaded. However, for those who can't be satisfied with religion, philosophy, money or materialism, or passing happiness; for those who doubt why life is such a suffering, or who we are, where we come from and where we go, or what is life and death; for those who suffer from not being able to find the answer to the above doubts, it would be essential to realize it.