The Buddha liked the practice of mindful breathing. And after he had become a Buddha, he continued the practice of mindful breathing. So, practicing mindful breathing was not exactly for the purpose of becoming a Buddha.
People say, "Thay does not teach much. He only teaches about breathing-in and breathing-out" and which is true. We only need to practice mindful breathing. That's enough.
Investigation and reflection have to do with thinking. And this is the positive thinking. This is the right thinking. By practicing right thinking, we have elements of wisdom. We can have access to the elements of wisdom. His mind will be calm and pure. And he will have perceptions leading to wisdom. And he will be able to bring his practice to completion. That is a kind of perception, right perception. They are called as parts of wisdom.
Perceptions conducive to parts of wisdom (明分想). So, he will acquire positive investigation and reflection that is about right thinking. He will be calm and pure. That is the effect of calming and purification. And he will have perceptions leading to wisdom. And he will be able to bring his practice to completion. So, the practice of mindful breathing has a very important place in the Buddhist teaching and practice.
I myself have gone through difficult moments in a war, a division of a country, the death of my mother, the difficulties within the Buddhist community. I was sick. I had a depression. And it is thank to the mindful breathing that I could overcome all these difficult moments. So, I have a very strong conviction in the effectiveness of the practice.
Thich Nhat Hanh