Diskpart a concurrent second call is made before the first is complete

[Brink]

http://www.sevenforums.com/tutorials/52129-disk-clean-clean-all-diskpart-command.html

I'd say that if you needed or wanted to do a more thorough wipe of the HDD to help make sure a virus is deleted or make sure that the data is secure erased better, then the "Clean All" command may be what you after.

If you just wanted to delete all partitions on a disk and are unable to do so in the Drive Options section during a normal clean install of Windows 7, then the "Clean" command at boot would be a faster option than "Clean All" if you didn't need to secure erase.

It's just a matter of what is needed in the circumstance and how thoroughly the HDD needs to be deleted.