Before I started learning Japanese, I had been learning English for many years. I knew that to progress I had to watch and read a lot of native content. I also knew from experience that classes and textbooks are entirely useless and don't help. I had an intuitive understanding of how one should learn a language, but I knew nothing about Krashen, AJATT and immersion learning.
Discovering AJATT helped me understand why what I did to learn English worked. It gave me a systemic view on the process of language acquisition and a framework to talk about immersion learning.