具有ValueDictionary锁sync.RwMutex的实现字典结构

I checked how I may implemented dictionary in go-lang,

Why we have https://flaviocopes.com/golang-data-structure-dictionary/?

type ValueDictionary struct {
    items map[Key]Value
    lock  sync.RWMutex
}

From official documentation we have

Package sync provides basic synchronization primitives such as mutual exclusion locks.

Why we used lock ?