Whenever somebody tells you about kanji, they say something like this:

Kanji are used for writing nouns, adjectives, adverbs and verbs.

However, the further I go, the more realize that this statement does not always hold true, and, in fact, I find people seemingly use whatever script they want at times. Let’s have a look at this line of lyric from 君がくれたもの


This is how you would find this lyric on Google and any other place. It’s almost always written like this. However, it makes no sense to me why 綺麗に is written as きれいに and 切なく is even written in カタカナ. This seems so random. I thought this should be the more standard version:


Even the title of the song uses もの instead of 物.

The more lyrics I read, the more I find this being a common practice. Even in the same song, 同じ and おなじ, わたし and 私 are both used at different places for seemingly no reason.

Can someone explain the reason behind this?