There are some sites for which the
www subdomain is not merely an alias for the base domain, and where you may wish to whitelist the
www subdomain without whitelisting all subdomains.
My specific use-case involves Tumblr: I had whitelisted uBlock Origin on
www.tumblr.com (where the user's posting interface lives, but where no public Tumblr page lives, all of them being on separate subdomains of
tumblr.com), because it was running slowly and I wanted to keep as many extensions as possible from running there while I was trying to edit a post. I do not, however, want to whitelist my ad-blocker on every single Tumblr page, which is what whitelisting
tumblr.com (without the
www) would do; when I enter
www.tumblr.com in the exceptions list, it is transformed to
tumblr.com, and it should not be.
I actually post to Tumblr very rarely, so I decided, instead, to just not whitelist Tumblr at all.