Why?
“You can never learn less; you can only learn more. The reason I know so much is because I have made so many mistakes.” – Buckminster Fuller
This chapter uses what you learned in the previous chapter (は) and adds a little to it: identity. You’ll be able to say “I” or “You” in relation to what you’re talking about. This further makes your noun-sentences more natural, and really sets the foundation for future grammar points with verbs, adjectives, and more.