Angular Using ngModel within a form, unnecessary to include one-way or two-way binding

In angular documentation https://angular.dev/api/forms/NgModel, it mentions

In the context of a parent form, it's often unnecessary to include one-way or two-way binding, as the parent form syncs the value for you
​

But I cannot understand why no need to include two-way binding? As this make the form just one way, from ui to ts.

e.g. https://stackblitz.com/edit/angular-5-starter-app-xzxmc6tp?file=app/app.component.ts,index.html

<form #f="ngForm" (ngSubmit)="onSubmit(f)" novalidate>
First Name<input name="first" ngModel required #first="ngModel" />
Last Name<input name="last" [(ngModel)]='lastName' />
<button>Submit</button>
<button type="button" (click)="updateLastName()">Change lastName </button>
</form>


in Last Name, I use [(ngModel)]='lastName', then I can modify the lastName property in ts and then shown in UI, if just use ngModel as in first name

First Name<input name="first" ngModel required #first="ngModel" />


then I cannot modify the first name in ts and shown in UI (even no a property link to UI First Name textbox).

So what good to omit [()]? why the document suggest

In the context of a parent form, it's often unnecessary to include one-way or two-way binding, as the parent form syncs the value for you ?
​

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