So what is katakana used for?

Katakana is mainly used to express foreign words in Japanese.

For example, Chicken is にわとり in Japanese. But if you want to express the English pronunciation of Chicken in Japanese writing, we will use katakana to write it as チキン(chi ki n).

And Foreign names are generally also written in Katakana.
For example, if the person name is Sara, we write サラ in Katakana.

Also we often use Katakana for onomatopoeia and to emphasize words. (ドキドキ = ba-dum ba-dum, heart pumping sound)