If you’re learning kanji, I assume you already know kana? If so, there’s not a good reason to use romaji and it should be avoided at all times!
In Japanese, readings for characters are listed in hiragana for kunyomi, and katakana for onyomi. I’m actually not sure why a textbook would be teaching kanji yet using romaji for the readings, while being aware they need to differentiate kunyomi/onyomi, and teaching a script that’s much more difficult to master than kana (which should be taught first, and is needed to read Japanese-- it’s also is very quick to master), that doesn’t seem very logical! Does the book also list the readings in hiragana/katakana, or do they only list romaji readings?
I highly recommend not studying individual kanji on their own unless it’s an actual word by itself, and instead add high frequency words that use the characters. That way you would naturally be adding words that both use the kun and onyomi readings, and as an added benefit learn how they are used in words while learning actual words. There’s a blog post we wrote about this I recommend reading, at: https://blog.skritter.com/2017/03/shouldnt-study-single-kanji-japanese-not-high-frequency-word/