Why doesn't DB_HOST need to be changed in wp-config.php when migrating?

At 5:15 in this vid, you can see that the define('DB_HOST', 'localhost'); statement remains unchanged. I'm confused why this wouldn't be manually changed as well, to something like 'studentshostingdatabases', or some other string provided by the hosting provider?

Background: I did the reverse of this video, that is, migrating a remote SQL db I've been playing with (through the first parts of this track) onto my local environment, to see if there were any hitches in the process.

In my remote wp-config file, this define statement had a very long url plugged in through the hosting SQL setup wizard when creating the original db (something like 'ftp.godaddy.123445678abc123'); noticing this, I changed it over in my local copy of the file to 'localhost' as that seemed pretty logical. Everything is working fine on my setup, at least as far as I can tell.

So, is this DB_HOST actually a necessary bit of info to update, or just a global that's being stored and presented for descriptive purposes somewhere? Ie, would Zac's wp-config file as it appears at 5:15 actually run fine? What differences would he notice if not?