Setting preferred domain, is 301 redirect necessary?

Google:

Specify a preferred domain:

On the Webmaster Tools Home page, click the site you want.
Under Site configuration, click Settings.
In the Preferred domain section, select the option you want.

Note: Once you’ve set your preferred domain, you may want to use a 301 redirect to redirect traffic from your non-preferred domain, so that other search engines and visitors know which version you prefer.

I set my preferred domain to “https://mydomain.com”. My WordPress Site Address is also set to “https://mydomain.com” (in the WordPress General Settings dashboard).

It seems like “https://www.mydomain.com” already redirects to “https://mydomain.com” without setting up any 301 redirect.

For seo, is it still necessary to set a 301 redirect as Google suggests?

Or is it enough just to set your preferred domain?