@lxo "Katakana is used for words brought in from other languages" is an oversimplification.
One, may loanwords (especially from Chinese) are rendered in kanji.
Two, many non-loanwords are written in katakana. For example, animal and plant species, as well as sound effects (onomatopeia). Katakana is also used to make a piece of text stand out, much like Western text would make text italic. See https://en.wikipedia.org/wiki/Katakana#Usage