For example, I:

1. Created a class with private Subject and WeakMaps to add Subject and Subscription

    const subjectsMap = new WeakMap();
    const subscriptionsMap = new WeakMap();
    
    class Test {
      #privateSubject = new BehaviorSubject(false);
      
      constructor() {
        subscriptionsMap.set(this.#privateSubject.subscribe(console.log), true);
        subjectsMap.set(this.#privateSubject, true);
      }
    }

2. Created a new Test instance

let test = new Test()

3. Checked my Weakmaps

console.log(subjectsMap)
> WeakMap {BehaviorSubject => true}

console.log(subscriptionsMap)
> WeakMap {SafeSubscriber => true}

4. Deleted the reference to my Test instance

test = null

5. Checked my WeakMaps and voila

console.log(subjectsMap)
> WeakMap {}

console.log(subscriptionsMap)
> WeakMap {}

My Subject and Subscription are destroyed. So why do I need to unsubscribe from them in the component?