In my opinion, contextual help is the best help.

Once the user is done typing and changes focus, showing a validation error and taking back users attention to the form element will not only be annoying, but also increase the form completion time.

However, we also need to ensure that we do not overdo helping 😜. Else the result would be otherwise. We need to have an understanding of our users and only provide assistance where its required.

Let me know your thoughts :)