When selecting you Cat Emotion, sometimes a single word or phrase just isn't enough. I find it helpful to occasionally write out my thoughts and feelings, so the "Why How" is a place for you to do that. Why are you feeling this way? How did you come to this emotion? The "Why How" section is a place for you to express those thoughts (if you want to).
I don't actually store everything written down, instead I only store the "keywords" that you use (effectively the adjectives and verbs you use).
If you're currious exactly how I do that, I use this library. And this is a link to my source code for how I use that library.