This is the modern usage of hiragana and katakana.
Earlier, e.g. in the books written a few hundred years ago, hiragana was used like today, i.e. inlined with kanji, for writing verb and adjective terminations or various grammatical particles, but katakana was used as furigana, for showing the pronunciation of the more seldom used or ambiguous kanji, especially when the Chinese reading was used for those kanji.
The use of katakana for the writing of foreign words has developed from its use for writing the approximate pronunciation of Chinese words.
The writing reform enforced after WWII has introduced significant changes in the use of hiragana and in the correspondences between kanji and hiragana (and also many kanji have been substituted with simplified variants), so reading a pre-war book can be challenging for someone familiar only with modern Japanese.
Earlier, e.g. in the books written a few hundred years ago, hiragana was used like today, i.e. inlined with kanji, for writing verb and adjective terminations or various grammatical particles, but katakana was used as furigana, for showing the pronunciation of the more seldom used or ambiguous kanji, especially when the Chinese reading was used for those kanji.
The use of katakana for the writing of foreign words has developed from its use for writing the approximate pronunciation of Chinese words.
The writing reform enforced after WWII has introduced significant changes in the use of hiragana and in the correspondences between kanji and hiragana (and also many kanji have been substituted with simplified variants), so reading a pre-war book can be challenging for someone familiar only with modern Japanese.