Why?
“Compile your to-do list for tomorrow no later than this evening.” – Timothy Ferriss
In this chapter you’re learning negative tense nouns. You know positive/future tense nouns, so now it’s time to learn negative. You’re learning this now because it’s step two of four when it comes to the noun tenses (present/future, negative, past, past-negative). This will also help solidify your knowledge (and expand your knowledge) of basic Japanese nouns grammar. It will also make it possible to learn past-negative (after learning past tense).