From the docs (emphasis mine):
A RWMutex is a reader/writer mutual exclusion lock. The lock can be held by an arbitrary number of readers or a single writer. The zero value for a RWMutex is an unlocked mutex.
In other words, readers don’t have to wait for each other. They only have to wait for writers that are holding the lock.
A sync.RWMutex is thus preferable for data that is mostly read, and the resource that is saved compared to a sync.Mutex is time.