For example you want to show something based on two async values. Assume, we have 2 Observables:
user$ = this.store.select(getUser);
product$ = this.store.select(getProduct);
We can show something based on both these values by using the nested ng-containers:
<ng-container *ngIf="user$ as user">
<ng-container *ngIf="product$ as product">
<div>We have {{product | json}} and {{user | json}}</div>
</ng-container>
</ng-container>
When product and user are both exist we will show the div.