When is Kanji REALLY used?


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

> 花火が夜空きれいに咲いて、ちょっとセツナク

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:

> 花火が夜空綺麗に咲いて、ちょっと切なく

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?

