Difference between shared mutex and mutex (why do both exist in C++ 11)?

By use of normal mutexes, you can guarantee exclusive access to some kind of critical resource – and nothing else. Shared mutexes extend this feature by allowing two levels of access: shared and exclusive as follows:

• Exclusive access prevents any other thread from acquiring the mutex, just as with the normal mutex. It does not matter if the other thread tries to acquire shared or exclusive access.
• Shared access allows multiple threads to acquire the mutex, but all of them only in shared mode. Exclusive access is not granted until all of the previous shared holders have returned the mutex (typically, as long as an exclusive request is waiting, new shared ones are queued to be granted after the exclusive access).

A typical scenario is a database: It does not matter if several threads read one and the same data simultaneously. But modification of the database is critical - if some thread reads data while another one is writing it might receive inconsistent data. So all reads must have finished before writing is allowed and new reading must wait until writing has finished. After writing, further reads can occur simultaneously again.

Edit: Sidenote:

Why readers need a lock?

This is to prevent the writer from acquiring the lock while reading yet occurs. Additionally, it prevents new readers from acquiring the lock if it is yet held exclusively.

"Shared mutexes are usually used in situations when multiple readers can access the same resource at the same time without causing data races, but only one writer can do so."

cppreference.com

This is useful when you need read/writer lock: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock