JSON is great… for computers. Even if it’s pretty-printed, trying to find relationships in the data is tedious. Human eyes, especially when looking at an ssh terminal, need compact and aligned text. The cat API aims to meet this need. All the cat commands accept a query string parameter help to see all the headers and info they provide, and the /_cat command alone lists all the available commands.
APIのドキュメントなのにこんなにフランクでいいんですかね……
catは人間が読みやすい形(おもにJSON)に変換して返却してくれる系のAPI。なんでCATなのかは知らない。indiceはよく使います。