What teachers and books tell you about Katakana is that it is generally used to write words that derive from foreign languages. Recently though, I came across a lot of cases where pure Japanese words were written with Katakana. E.g. a cover from Yui says "タイヨウのうた" instead of "太陽の歌". While I can understand that sometimes it is more casual to use Hiragana instead of Kanji, I cannot understand why Katakana is used here. Is it considered "cooler"?