Filesystems often have a 'dirty' flag which is cleared as the very last write when unmounting cleanly, to let the OS know whether the filesystem might need to b

Filesystems often have a 'dirty' flag which is cleared as the very last write when unmounting cleanly, to let the OS know whether the filesystem might need to be checked before mounting it next time. Also, unmounting helps you to be confident that nothing's started doing more IO since you ran sync.