There was a time when the sync CLI exited before the work was done, so you actually didn't want sync;sync;halt, typing it as separate commands gave it the right amount of time to complete.

Or maybe three times sync;sync;sync just to make sure :)

I know of a company whose sysadmins still put sync;sync;sync in the scripts they deploy on customers’ machines… just because “you never know”.

Don't forget to park the hard drive heads!