

Here are some examples of where katakana is used in Japanese writing:
loanwords from other languages:ビル (‘building’), コーヒー (‘coffee’), クロワッサン (‘croissant’)
foreign names, including people and places:ジョン スミス (‘John Smith’), ロンドン (‘London’)
onomatopoeia and sound effects, including in manga:ピンポン (‘pinpon’), the “ding-dong” sound of a doorbell
for emphasis, similar to italics in English
plant and animal names in scientific writing