Here are some examples of where katakana is used in Japanese writing.
Foreign loan words:ビル (‘building’), コーヒー (‘coffee’), クロワッサン (‘croissant’)
Foreign proper nouns:ロンドン (‘London’), ジョン スミス (‘John Smith’)
Onomatopoeia (words used to represent sounds), used heavily in spoken and written Japanese, including manga:ピンポン (‘pinpon’), the “ding dong” sound of a doorbell.
For emphasis, similar to the use of italics in English.
For clarity, similar to the use of capital letters in English.