What is the function of the dep of the observer part of the vuejs source code

observer class: 

constructor (value: any) {
    this.value = value
    this.dep = new Dep()
    this.vmCount = 0
    def(value, "__ob__", this)
    if (Array.isArray(value)) {
      const augment = hasProto
        ? protoAugment
        : copyAugment
      augment(value, arrayMethods, arrayKeys)
      this.observeArray(value)
    } else {
      this.walk(value)
    }
  }

what is the purpose of the dep attribute here?
I know that the defineReactive method modifies the getter setter method to collect dependencies and notify watcher, but this method is operated by dep in the closure, so why declare an instance of dep in observer?

Vue.js
Mar.21,2021
The

dep attribute is used in Vue.$set and $delete. For example, the penultimate line of code in $set 

export function set (target: Array<any> | Object, key: any, val: any): any {
  if (process.env.NODE_ENV !== 'production' &&
    (isUndef(target) || isPrimitive(target))
  ) {
    warn(`Cannot set reactive property on undefined, null, or primitive value: ${(target: any)}`)
  }
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.length = Math.max(target.length, key)
    target.splice(key, 1, val)
    return val
  }
  if (key in target && !(key in Object.prototype)) {
    target[key] = val
    return val
  }
  const ob = (target: any).__ob__
  if (target._isVue || (ob && ob.vmCount)) {
    process.env.NODE_ENV !== 'production' && warn(
      'Avoid adding reactive properties to a Vue instance or its root $data ' +
      'at runtime - declare it upfront in the data option.'
    )
    return val
  }
  if (!ob) {
    target[key] = val
    return val
  }
  defineReactive(ob.value, key, val)
  ob.dep.notify() // depdep
  return val
}

Dep is an observer mode when the Watcher is added to the sub array when the class, is initialized, and the notify, of the Dep is triggered to perform the Watcher,vue update view when the data changes

.

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