std::map/std::set and equal_range(): what's the reasoning here?

I just noticed that std::map and std::set have the member function equal_range() returning an iterator range of values for a certain key. How does this make sense when std::map and std::set are always ordered key/value (as in a single key for a single value, or both combined) containers.

I would expect this member function with std::multimap and std::multiset since both of these allow for multiple values sharing the same key, and of course they both do. What am I missing?

It makes sense as conformity with other associative container. Similarly std::map and std::set have a count method even though they can only return 0 or 1 and find does the job already. You can use count with associative container no matter if they can have more than 1 same element (cf. std::multi_map). And you can use equal_range for a std::unordered_map in the same way as you can use it for a ordered std::map. You are right that for a std:map and std::set those are rarely useful, but suppose you write generic code that accepts an associative container (ordered or not):

template <typename T,typename E>
void foo(T& t,E e) {
     auto p = t.equal_range(e);
     //...
}

There is no formal requirement that all associative container must have equal_range but since c++20 you can write a concept that requires T must have equal_range and use the template foo also with std::map.