Whenever a lot of items shall be stored in a sorted manner, and the key by which they are sorted can occur multiple times, std::multimap is a good choice.

Let's find an example use case: When writing text in German, it is okay to use very long sentences. When writing texts in English, it is not. We will implement a tool that helps German authors to analyze their English text files, focusing on the length of all sentences. In order to help the author in improving the text style, it will group the sentences by their length. This way the author can pick the longest ones and break them down.