What is the usage of Angular directive novalidate in a form html tag

You can use the same ones used by the HTML 5 specification in Angular,so you can add the novalidate attribute to the form element, which tells the browser not to use its native validation. Because different browsers have different implementation validations. Since Angular get validation itself, the browser don't need to do validation implementation.

novalidate attribute is used to disable browser's native form validation.

You can use it when you need do your own AngularJS custom validation.

It prevents the browser's native validation to kick in i.e form data will not be validated upon submission. Examples include input where type='email'

Note that it is not Angular's directive. It is HTML 5 attribute. Read more about it here