This issue has been thoroughly aired here: Diskpart Clean All debate thread

There is nothing that using Diskpart to write zeroes does to "clean" a drive other than in certain cases where the boot sector may have corrupt or infected code which needs overwriting for install to proceed, for which the "clean" command works just as well.

The only other benefit is to make data unrecoverable by covering it all up with zeroes using "Clean all". This is not "cleaning" the HD but destroying data by making it unreadable.