Windows permissions are more flexible than basic Unix ones. A file doesn’t just have an owner and a group, it can have individual permissions for arbitrarily many entities, so taking ownership doesn’t remove any of the permissions from anything that already had access, it just adds more. The command shown here is closest in effect to deciding you’re always going to log in as root from now on, although Windows has a way to effectively do that without modifying the ACL of every file. Either way, it’s silly, and usually people who suggest it are under the impression that XP did permissions right by not meaningfully enforcing them and not having an equivalent of a root account you can temporarily switch to, and Vista only changed things specifically to annoy people, and not to be more like Unix.