It's also good from a security perspective.

Anyone who can upload HTML pages to subdomain.domain.com can read and write cookies for *.domain.com, unless you declare yourself a public suffix and enough time has passed for all the major browsers to have updated themselves.

I've seen web hosts in the wild who could have their control panel sessions trivially stolen by any customer site. Reported the problem to two different companies. One responded fairly quickly, but the other one took several years to take any action. They eventually moved customers to a separate domain, so the control panel is now safe. But customers can still execute session fixation attacks against one another.